scsi-disk: no need to call scsi_req_data on a short read

scsi-disk: no need to call scsi_req_data on a short read

In fact, if the HBA's transfer_data callback goes on with scsi_req_continue
the request will be completed successfully instead of showing a failure.
It can even cause a segmentation fault.

An easy way to trigger it is "eject -f cd" during installation (during media
test if the installer does something like that).

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