scsi-disk: bump SCSIRequest reference count until aio completion runs

scsi-disk: bump SCSIRequest reference count until aio completion runs

In some cases a request may be canceled before the completion callback
runs.  Keep a reference to the request between starting an AIO operation
and the corresponding scsi_req_cancel or scsi_*_complete.

When a request has to be retried, the request can be dropped because
scsi_dma_restart_bh only looks at requests that are enqueued.  As such,
they always have at least a reference.

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