always teardown snapshot's memory volume

Teardown the memory volume of snapshot with memory as part of 'finally'
block to ensure it is done in case an exception is thrown in the
'snapshot' method as well.

Change-Id: If59047f7d5ba6cfa942b593683f6f9987619a7ea
Signed-off-by: Arik Hadas <ahadas@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26544
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>