blkverify: Don't require protocol filename

blkverify: Don't require protocol filename

If the filename is not prefixed by "blkverify:" in
blkverify_parse_filename(), the blkverify driver was not selected
through that protocol prefix, but by an explicit command line (or QMP)
option (like driver=blkverify).

If blkverify_parse_filename() has been called, a filename has been
given. If it is not prefixed, it is probably really just a plain
filename. This is no problem, since we can use it as the test image
filename and rely on the user to specify the raw image filename through
the new corresponding option.

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