scsi: introduce SCSIReqOps This will let allow requests to be dispatched through different callbacks, either common or per-device. This patch adjusts the API, the next one will move members to SCSIReqOps. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>