vm: janitorial: detect if dom is running

this patch extracts a little helper method
to detect if the underlying libvirt
domain is running.

Change-Id: If5bd28c43ab934dc5525fc952b646e2bd9b71b78
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28510
Reviewed-by: Dan Kenigsberg <danken@redhat.com>