qemu-io: add -r option to register I/O buffer

The blk_register_buf() API is an optimization hint that allows some
block drivers to avoid I/O buffer housekeeping or bounce buffers.

Add an -r option to register the I/O buffer so that qemu-io can be used
to test the blk_register_buf() API. The next commit will add a test that
uses the new option.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230207203719.242926-4-stefanha@redhat.com>