core: move the logic in VmPoolHandler to separate command

The logic of how to handle VM that went down is extracted to separate
command, so the invocation of this logic is be in a more standard way.
The DownVmsHandler remains, it will later on be used as a listener for
notifications from VURTI.

Change-Id: Ic8840b8cba0fcec68936f0f24abc9948568cca0d
Bug-Url: https://bugzilla.redhat.com/1098791
Signed-off-by: Arik Hadas <ahadas@redhat.com>