The throttle driver is not a protocol so it should implement bdrv_open
instead of bdrv_file_open and not provide a protocol_name.

Attempts to invoke this driver using protocol syntax
(i.e. throttle:<filename:options:...>) will now fail gracefully:

  $ qemu-img info throttle:foo
  qemu-img: Could not open 'throttle:foo': Unknown protocol 'throttle'
Signed-off-by: NFabiano Rosas <farosas@linux.vnet.ibm.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>