The previous behaviour was to finish AIOCBs inside curl_aio_readv()
if the data was cached. This caused the following failed assertion
at hw/ide/pci.c:314: bmdma_cmd_writeb

"Assertion `bm->bus->dma->aiocb == ((void *)0)' failed."

By scheduling a QEMUBH and performing the completion inside the
callback, we avoid this problem.
Signed-off-by: NNick Thomas <nick@bytemark.co.uk>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>