blkdebug: Don't require sophisticated filename

blkdebug: Don't require sophisticated filename

If the filename is not prefixed by "blkdebug:" in
blkdebug_parse_filename(), the blkdebug driver was not selected through
that protocol prefix, but by an explicit command line option
(file.driver=blkdebug or something similar). Contrary to the current
reaction, this is not a problem at all; we just need to store the
filename (in the x-image option) and can go on; the user just has to
manually specify the config option.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>