Fix parent volume rollback after failure to create snapshot

1. Calling teardown() without prepare() throws an exception
2. Narrowing try/except block
3. Moving call to teardown() to finally block
4. Remove redundant internal "if"

Change-Id: I11948aacad21008ffbb4cb23013bf363e60bdb9b
Bug-Url: https://bugzilla.redhat.com/982004
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18205
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>