RBD: Add support readv,writev for rbd

Rbd can do readv and writev directly, so wo do not need to transform
iov to buf or vice versa any more.

Signed-off-by: tianqing <tianqing@unitedstack.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>