Reverts with the LVM driver were happening asynchronously
which resulted in unreliable behavior. This change makes
the reverts synchronous.
Also, the driver now unexports and unmounts both the share and
the snapshot during the revert (required to make the operation
synchronous) which requires changing the driver interface to
include the snapshot access rules in addition to the share access
rules.
Closes-bug: #1707943
Change-Id: Ifea1799e1d2e3963fec7e90ce3f9cb47b9f02f4f
5cb87d5dc3