hw/block: Use errp directly rather than local_err

[Drop virtio_blk_data_plane_create() change that misinterprets return
value when the virtio transport does not support dataplane.
--Stefan]

Cc: John Snow <jsnow@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>

Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: e77848d3735ba590f23ffbf8094379c646c33d79.1511317952.git.maozy.fnst@cn.fujitsu.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>