block/null: Remove 'filename' option

This option was only added to allow 'null-co://' and 'null-aio://

' as
filenames, its value never served any actual purpose and was ignored.
Nevertheless it was accepted as '-drive driver=null,filename=foo'.

The correct way to enable the protocol prefixes (and that without adding
a useless -drive option) is implementing .bdrv_parse_filename. This is
what this patch does.

Technically, this is an incompatible change, but the null block driver
is only used for benchmarking, testing and debugging, and an option
without effect isn't likely to be used by anyone anyway, so no bad
effects are to be expected.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>