This patch keep the recursive way of doing things but simplify it by giving
two responsabilities to all block filters implementors.

They will need to do two things:

-Set the is_filter field of their block driver to true.

-Implement the bdrv_recurse_is_first_non_filter method of their block driver like
it is done on the Quorum block driver. (block/quorum.c)

[Paolo Bonzini <pbonzini@redhat.com> pointed out that this patch changes
the semantics of blkverify, which now recurses down both bs->file and
s->test_file.
-- Stefan]
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>