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.

vrtManager/tests/test_host_reads.py

@@ -21,6 +21,7 @@ from libvirt import (
     VIR_ERR_NO_STORAGE_VOL,
     VIR_ERR_NO_SUPPORT,
     VIR_ERR_OPERATION_DENIED,
+    VIR_ERR_SYSTEM_ERROR,
     VIR_ERR_XML_ERROR,
     libvirtError,
 )
@@ -136,13 +137,16 @@ class ChangeInterfacesTestCase(unittest.TestCase):
 
     def test_a_backend_without_define_cannot_change_interfaces(self):
         conn = connection()
-        # any other refusal (access denied, ...) leaves the page as it was
-        for code, changeable in ((VIR_ERR_NO_SUPPORT, False), (VIR_ERR_XML_ERROR, True), (VIR_ERR_OPERATION_DENIED, True)):
+        for code, changeable in ((VIR_ERR_NO_SUPPORT, False), (VIR_ERR_OPERATION_DENIED, False), (VIR_ERR_XML_ERROR, True)):
             with self.subTest(code=code):
                 conn.wvm.interfaceDefineXML = MagicMock(side_effect=self.error(code))
                 self.assertIs(conn.can_change_interfaces(), changeable)
                 # an invalid definition: nothing can be defined by asking
                 conn.wvm.interfaceDefineXML.assert_called_once_with("<interface/>", 0)
+        # any other refusal (the connection, the login) is no answer
+        conn.wvm.interfaceDefineXML = MagicMock(side_effect=self.error(VIR_ERR_SYSTEM_ERROR))
+        with self.assertRaises(libvirtError):
+            conn.can_change_interfaces()
 
 
 class CapacityTestCase(unittest.TestCase):