core: minor refactorings in StopVmCommandBase

- rearrange stopSuspendedVm method to be more readable
- rename Destroy method to destroyVm
- remove usages of DbFacade class

Change-Id: Ie347eb6b9df6284038dd5b9028f3d39d81e04f64
Signed-off-by: Arik Hadas <ahadas@redhat.com>