vm: make _isDomainRunning more robust

The Vm._dom atrribute may be set to None
asynchronously by the _onQemuDeath callback.

This patch makes sure that the
Vm._isDomainRunning method can cope with this fact.

Change-Id: Idf58cc3a90fd85b858afba4f86f275b4059fe3e7
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30139
Reviewed-by: Dan Kenigsberg <danken@redhat.com>