utils: Moved RollbackContext from misc to utils

The RollbackContext class was located in storage/misc, and is used by
several non-storage tests. It was therefore moved to utils, where it
belongs. Accordingly, RollbackContextTests class was moved to utilsTests
from miscTests.

Change-Id: I985103650a5706d35d9cd519618d09c692feb0be
Signed-off-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22861
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>