GlusterFS: Use correct base argument when deleting attached snaps

When deleting the most recent snapshot, the 'file_to_merge' field
which translates into the base= field for libvirt's blockRebase
call in Nova must be set depending on whether other snapshots exist.

If there are no other snapshots, base = None, which results in
libvirt clearing the qcow2 backing file pointer for the active
disk image.

If there are other snapshots, pass the parent of the file being
deleted as the new base file.  The snapshot info pointer for the
prior base file must also be updated in this case.

TODO: tests

Change-Id: If7bc8259b031d0406346caafb8f688e65a38dba6