core:Create snapshot will be blocked on hibernate.

VM which is in hibernate process, will not be able to create live
snapshot.

The proposed fix is to block create snapshot process when VM is in
saving or restoring state.

Change-Id: I286ca81b157062feb6ff77a96936c0d0dba87b2a
Related-to: https://bugzilla.redhat.com/877978
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>