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/tests/test_audit_rows.py

@@ -21,7 +21,8 @@ class InterfaceRowTests(TestCase):
     def test_interface_actions(self, ifaces, iface):
         ifaces.return_value.can_change_interfaces.return_value = True
         ifaces.return_value.get_net_devices.return_value = ["eth0"]
-        ifaces.return_value.get_ifaces.return_value = []
+        ifaces.return_value.get_member_candidates.return_value = ["eth0"]
+        ifaces.return_value.get_ifaces_info.return_value = []
         iface.return_value.can_change_interfaces.return_value = True
         self.client.post(reverse("interfaces", args=[self.compute.id]), {
             "create": "", "name": "br1", "itype": "bridge", "start_mode": "onboot", "netdev": "eth0",