Volume IO usage gets reset to 0 after a reboot / crash

Check for reset of block device stats before updating volume usage cache.

If an instance is reboot or crashes, the results of block device stats for
that domain get reset. Add check that if block device stats are lower than
the current stats in the database, then add current values to totals and
update.

Change-Id: I3011872f8d80a1519771f8423001650dcbcc7f5b
Fixes bug: 1169097
(cherry picked from commit a4cb456631f0e70685fbc361eced9baa477a8c56)
Merge "Volume IO usage gets reset to 0 after a reboot / crash" into stable/grizzly