virtio-blk: handle NULL returns from bdrv_aio_{read, write}

virtio-blk: handle NULL returns from bdrv_aio_{read, write}

The bdrv_aio_{read,write} routines can return a NULL pointer when the
I/O submission fails.  Currently we ignore this and will wait forever
for an I/O completion and leading to a hang of the guest.

I can easily reproduce this using the native Linux AIO patch, but it's
also possible using normal pthreads-based AIO.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>