core: minor refactoring

* Rename RunVmCommandBase#decreasePendingVms to decreasePendingVm
* Rename VmHandler#decreasePendingVms(VmStatic, Guid) to
  decreasePendingVm
* Change VmHandler#decreatePendingVms(Guid, int, int, String) to be
  private
* Rename VmHandler#decreatePendingVms(Guid, int, int, String) to
  decreatePendingResources

Change-Id: I04c00f9b4f6271a4662199097d4ceff3e0d3f068
Signed-off-by: Arik Hadas <ahadas@redhat.com>