fix(interfaces): interface changes roll back as one, a running one is not deleted, each interface is read once

Plan reviewed with Codex; tried on a udev host (AlmaLinux 10) and a
netcf one (AlmaLinux 8).

- Changes are one change of the host's configuration (libvirt's
  interface change transaction): a bridge that does not start used to
  be undefined, leaving its member without the addresses and start
  mode the bridge took from it; now the whole change rolls back, the
  member's own configuration too, and so does a commit libvirt refuses.
  A rollback that fails too is reported with the failure, saying the
  host's configuration may be changed.
  Deleting a bridge, which defines its
  members again on their own, is one change as well.
- A running interface is not deleted: the page hid Delete, but a post
  deleted it. A bridge with addresses and several members is refused
  (which member takes the addresses is the user's choice); the
  confirmation says that its members are defined again.
- A bridge takes only a device in no bridge, bond or VLAN as the host
  has them configured, at any depth (a bridge's bond's devices), not
  the loopback, a libvirt network's bridge or a VM's tap; an ethernet
  interface configures only such a device too.
- Whether the host can change interfaces: unsupported or refused by
  its access rules is a no, bad XML a yes, anything else (the
  connection, the login) is raised instead of taken for a yes. A page
  whose host could not be read offers no actions.
- Reads: the list reads each interface's XML once from one listing
  (listAllInterfaces: every state, not the active and the defined ones
  apart); a page reads the running and the saved definition once each
  (cached_property). netcf writes a link speed of -1 for a card that
  has none (a virtio one) and then refuses its own XML: the saved one
  is read instead, where virsh and the page failed. Name and MAC come
  from libvirt's object, without a call. Each family says how it is
  addressed (DHCP, autoconf, static or none) and every address, its
  prefix where it has one; no
  address is not "DHCP" any more. A bridge's members carry their link
  state and speed; an unknown speed (0, -1, 4294967295) is none.
- The interface API lists from the same reader. get_details,
  get_bridge, get_bridge_slave_ifaces and the per-family getters go.

interfaces/forms.py

@@ -32,12 +32,14 @@ class AddInterface(forms.Form):
     stp = forms.ChoiceField(required=False, choices=(("on", "on"), ("off", "off")))
     delay = forms.IntegerField(required=False, min_value=0)
 
-    def __init__(self, *args, netdevs=(), defined=(), **kwargs):
-        # The host's network devices: a bridge member, or the device an
-        # ethernet interface configures, must be one of them. The interfaces
-        # the host defines: defining one again would replace its configuration.
+    def __init__(self, *args, netdevs=(), members=(), defined=(), **kwargs):
+        # The host's network devices. members: those a bridge can take
+        # (wvmInterfaces.get_member_candidates); an ethernet interface
+        # configures one of them too. The interfaces the host defines:
+        # defining one again would replace its configuration.
         super().__init__(*args, **kwargs)
         self.netdevs = set(netdevs)
+        self.members = set(members)
         self.defined = set(defined)
 
     def clean_ipv4_addr(self):
@@ -122,8 +124,10 @@ class AddInterface(forms.Form):
                 self.add_error("netdev", _("A bridge needs a device"))
             elif netdev and netdev not in self.netdevs:
                 self.add_error("netdev", _("The device is not on this host"))
+            elif netdev and netdev not in self.members:
+                self.add_error("netdev", _("The device is a bridge, bond or VLAN, a member of one, or a VM's"))
             elif netdev and netdev == name:
                 self.add_error("netdev", _("A bridge cannot contain itself"))
-        elif cleaned.get("itype") == "ethernet" and name and name not in self.netdevs:
-            self.add_error("name", _("An ethernet interface configures a device of this host"))
+        elif cleaned.get("itype") == "ethernet" and name and name not in self.members:
+            self.add_error("name", _("An ethernet interface configures a device of this host in no bridge, bond or VLAN"))
         return cleaned