core: move VM status handling to VdsManager.

VMs get stuck in 'Unknown' state when power management is not working

Move the VM state change to 'UnKnown' done in
VdsNotRespondingTreatmentCommand::MoveVMsToUnknown to VdsManager code

The only change is that in order to prevent nested locks we should run
the call to destroyVmOnDestination(vm) method in a separate thread and
then we will have two independent locks.

Change-Id: I7304109b49a0fb8441f9fdc9aee6edb0c654ab0e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1064860
Signed-off-by: Eli Mesika <emesika@redhat.com>