Add io_uring value to capability replies.

The capability QEMU_CAPS_AIO_IO_URING will be used for io_uring aio mode,
introduced from QEMU 5.0, linux 5.1.
Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>