During an assisted volume snapshot delete request from Cinder nova
removes the snapshot from the backing file chain. During that nova
checks the existence of such file. However in some cases (see the bug
report) the path is relative and therefore os.path.exists fails.
This patch makes sure that nova uses the volume absolute path to make
the backing file path absolute as well.
Closes-Bug #1885528
Change-Id: I58dca95251b607eaff602783fee2fc38e2421944