blkdebug: silence warning under qtest

make check always outputs warnings, this
is not nice.  Disable blkdebug warnings under qtest.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1448883874-17933-1-git-send-email-mst@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
