core: rename RunVmCommandBase#processVmPoolOnStopVm

This method handles not only VMs from pool, but every VM that went down.
Thus, its name is changed to processVmOnDown

Change-Id: Ide45cda8a4fa5bb989d1a9eb8408717a9b071860
Signed-off-by: Arik Hadas <ahadas@redhat.com>