block/curl: Check protocol prefix

If the user has explicitly specified a block driver and thus a protocol,
we have to make sure the URL's protocol prefix matches. Otherwise the
latter will silently override the former which might catch some users by
surprise.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170331120431.1767-3-mreitz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>