Remove internal use of vircmd

Remove internal use of vircmd

It is too easy to clobber/interact with domains that are no specified
in the cluster configuration.

Replace use of vircmd with new function virsh_cluster().  The only
possible loss here is that unmatched nodes may not be destroyed, so
duplicate IP addresses might occur when cluster configuration changes.
To try to avoid this a warning is printed by virsh_cluster() when
domains that are not part of the cluster are found to match
CLUSTER/CLUSTER_PATTERN.

Signed-off-by: Martin Schwenke <martin@meltin.net>