core: fix NPE in decreasePendingVms

There was a race between the VdsUpdateRunTimeInfo thread that invokes
RunVmCommandBase#decreasePendingVms method through OnPowerringUp
callback and the thread that invokes MigrateVmCommand#rerun (the thread
is created in VdsEventListener#rerun). The 'rerun thread' was doing
'setVm(null)' in order to refresh the VM on the next getVm invocation
and the VdsUpdateRunTimeInfo thread was trying to the get the VM using
the 'getVm' method VM in the same time - as a result of this race, the
VdsUpdateRunTimeInfo thread might end up with null VM which results in
NPE.

The solution is to change the VdsUpdateRunTimeInfo thread to fetch the
VM directly from the DB instead of using the 'getVm' method, And since
we don't need the dynamic info of the VM we can query only the static
part of the VM.

Change-Id: Icaca2a3849a9fefbc33b055c6f4d2dd32959ad28
Bug-Url: https://bugzilla.redhat.com/1000824
Signed-off-by: Arik Hadas <ahadas@redhat.com>