scsi: pass cdb already to scsi_req_new

Right now the CDB is not passed to the SCSIBus until scsi_req_enqueue.
Passing it to scsi_req_new will let scsi_req_new dispatch common requests
through different reqops.

Moving the memcpy to scsi_req_new is a hack that will go away as
soon as scsi_req_new will also take care of the parsing.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
