core: skip execution of stop vm if its down

if vm is down, instead of failing in canDoAction,
skip execution and notify the user in audit log.

Change-Id: I75925d528cca52603660d5a7ec6c5c871304f3d0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1168304
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>