qemu: support live change of the bridge used by a guest network device

This patch was created to resolve this upstream bug:

  https://bugzilla.redhat.com/show_bug.cgi?id=784767

and is at least a partial solution to this RHEL RFE:

  https://bugzilla.redhat.com/show_bug.cgi?id=805071

Previously the only attribute of a network device that could be
modified by virUpdateDeviceFlags() ("virsh update-device") was the
link state; attempts to change any other attribute would log an error
and fail.

This patch adds recognition of a change in bridge device name, and
supports reconnecting the guest's interface to the new device.
Standard audit logs for detaching and attaching a network device are
also generated. Although the current auditing function doesn't log the
bridge being attached to, this will later be changed in a separate
patch.