Commit a4b15a8b introduced a new function blk_pread_nonzeroes(). Instead
of reading directly from the root node of the BlockBackend, it reads
from its 'file' child node. This can happen to mostly work for raw
images (as long as the 'raw' format driver is in use, but not actually
doing anything), but it breaks everything else.

Fix it to read from the root node instead.

Fixes: a4b15a8b
Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230307140230.59158-1-kwolf@redhat.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>