vdsm: Ignore after_vm_cont hook if _dom was not yet created

There's a race condition happening which results in an exception because the
domain has not yet being assigned/created.

This is a workaround to fix the exception from happening, however this is not
a long term solution.

Change-Id: I0e36f988c965b51855861f04bcebee5ad51d3b09
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12558
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>