core: fix update VmDevices when VM is down

Updating VmDevices when the VM isn't running,
mistakenly leads to next run snapshot creation.

Therefore, modified 'VmHandler -> isUpdateValidForVmDevices':
the function should check whether updating is valid according
to the status of the VM; hence, VmStatus should be specified
(so it could be validated properly on
'getVmDevicesFieldsToUpdateOnNextRun -> isUpdateValidForVmDevice').

Change-Id: Ia7a8f2a4c3f41b18a7c02ca162b01ee352e49bcf
Signed-off-by: Daniel Erez <derez@redhat.com>