core: fix for memory being removed from stateless snapshot

This patch changes the memory removal on RunVmCommand so that it will be
removed from the active snapshot instead of removing it from the
stateless snapshot for a VM that runs in stateless mode.

Change-Id: Ib4d2db4a5a21217b7ffce9a634524e196064276e
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>