core: don't allow snapshot for suspended vm

creating snapshot while vm is suspended is problematic,
since when resuming, vdsm still use the old volumes,
another problem is that the hibernation volumes info is not saved in the
ovf.

Change-Id: Icf2c4c3ec3561df1d35f5e4da964664d63598434
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120232
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>