vm: set self._dom to None after destroying vm

When receiving 'vmDestroy' command, we deal with it using the following
steps:
* invoke VM::releaseVm() in VM::destroy
* invoke _dom.destroyFlags()/_dom.destroy() to destroy vm in VM::releaseVm()
* When libvirt is done for destroying vm, it emits VIR_DOMAIN_EVENT_STOPPED
_SHUTDOWN/DESTROYED(due to different flows)
* VDSM receives the signal, it invokes VM::_onQemuDeath().
* invoke VM::releaseVm() in VM::_onQemuDeath().
Unfortunately, it doesn't set self._dom to None after destroying vm in
VM::releaseVm(). So, it cause the following libvirterror when invoking
VM::releaseVm() in VM::_onQemuDeath():

libvirtconnection::101::libvirtconnection::(wrapper) Unknown libvirterror: ecode:
42 edom: 10 level: 2 message: Domain not found: no domain with matching uuid
'1813cf7d-d62b-4cc2-86fc-de469e0e37eb' (xxx)

To ignore the libvirterror, the patch will set self._dom to None when getting signals
(VIR_DOMAIN_EVENT_STOPPED_SHUTDOWN/DESTROYED).

Change-Id: I4eca4c14e44a2a95628bf8e45a7a0a64d56446b8
Signed-off-by: jarod.w <work.iec23801@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/23014
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>