core: Lock VM on hibernate process.

Race between live snapshot and VM hibernate,
cause results which might be undefined.
The proposed fix is adding hibernateVM lock on the VM to avoid this
behaviour.

Change-Id: I3d35780a00a66fc0cc8708af2bc4cb6d292446f0
Related-to: https://bugzilla.redhat.com/877978
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>