When the function no success value to transmit, it usually make the
function return void. It has turned out not to be a success, because
it means that the extra local_err variable and error_propagate() will
be needed. It leads to cumbersome code, therefore, transmit success/
failure in the return value is worth.

So fix the return type of blkconf_apply_backend_options(),
blkconf_geometry() and virtio_blk_data_plane_create() to avoid it.

Cc: John Snow <jsnow@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: ac0edc1fc70c4457e5cec94405eb7d1f89f9c2c1.1511317952.git.maozy.fnst@cn.fujitsu.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>