blockdev: Remove 'media' parameter from blockdev_init()

The remaining users shouldn't be there with blockdev-add and are easy to
move to drive_init().

Bonus bug fix: As a side effect, CD-ROM drives can now use block drivers
on the read-only whitelist without explicitly specifying read-only=on,
even if a format is explicitly specified.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>