virtio-blk: Check return value of blk_aio_ioctl

Since commit 1dc936aa84 (virtio-blk: Use blk_aio_ioctl) we silently lose
the request if blk_aio_ioctl returns NULL (not implemented).

Fix it by directly returning VIRTIO_BLK_S_UNSUPP as we used to do.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[ kwolf: Fixed build error on win32 ]

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
