It appears possible to bring down a machine's network semi-permanently
by performing remote virt-manager tasks.

Specifically, the operation of creating a br0 interface based upon an
active ethN one appears to be fragile, if performed via a remote 
ssh/X session that used the ethN interface as its transport.  The gui
freezes midway during the change.  This in turn appears to stop the
interface reconfiguration in progress.  That in turn leaves the machine
with no live interfaces at all!

The underlying problem appears to be that the GUI progress display
and the low-level /etc/sysconfig/network-scripts* manipulation are
tied together too much.  virt-manager should do the changes in some
separate process, so even if the gui is hung, the changes will be
completed, as if it were an atomic operation.