raw-posix: Deprecate host floppy passthrough

raw-posix: Deprecate host floppy passthrough

Raise your hand if you have a physical floppy drive in a computer
you've powered on in 2015.  Okay, I see we got a few weirdos in the
audience.  That's okay, weirdos are welcome here.

Kidding aside, media change detection doesn't fully work, isn't going
to be fixed, and floppy passthrough just isn't earning its keep
anymore.

Deprecate block driver host_floppy now, so we can drop it after a
grace period.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>