sandbox: scsi: Move request-handling code to scsi_emul

Move this code into the emulator file so it can be used by multiple
drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/scsi/Makefile b/drivers/scsi/Makefile
index 25194ee..d1b40c6 100644
--- a/drivers/scsi/Makefile
+++ b/drivers/scsi/Makefile
@@ -17,4 +17,5 @@
 
 ifdef CONFIG_SCSI
 obj-$(CONFIG_SANDBOX) += sandbox_scsi.o
+obj-$(CONFIG_SANDBOX) += scsi_emul.o
 endif