scsi-disk: split scsi-disk reqops

Only checks for present medium were still done in scsi_send_command
for emulated commands.  So move those to scsi_disk_emulate_command
and return different SCSIReqOps depending on the kind of command.

Checks for present medium can be done unconditionally for the
scsi_disk_dma_reqops case.

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