vm: fix _dom access when QEMU is dying

commit 8fedf8bde3c28edb07add23c3e9b72681cb48e49 introduced a tiny
window for races between libvirt notifications (vm._onQemuDeath)
and polling for stats from engine.

This is demonstrated by bz1073478, but it really boils down
to how events are serialized.
In this case, a stats request from engine is being answered
while QEMU is being reported down and the _dom handle is being
shut down, we end up with an uncaught exception.

This patch address the reported race between polling and qemu death.
The window of vulnerability is (supposed to be) tiny
and the issue itself is hard to reproduce.

Sampling thread is theorically vulnerable to the a race of the
same type, but this problem will be addressed in a separate
patch.

Change-Id: Ibac9e75a484b228c67d5432b71e965bdf317b89c
Bug-Url: https://bugzilla.redhat.com/1073478
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26539
Reviewed-by: Dan Kenigsberg <danken@redhat.com>