usb: Update the test to cover reading and writing

Add test coverage for blk_write() as well.

The blk_erase() is not tested for now as the USB stor interface does not
support erase.

Signed-off-by: Simon Glass <sjg@chromium.org>

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>

sandbox: scsi: Move file size into shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

sandbox: scsi: Move block size into shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

scsi: Move vendor/product info into the shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

sandbox: Move buffer to scsi_emul_info

Move the buffer into this struct so it can be shared between different
implementations.

Signed-off-by: Simon Glass <sjg@chromium.org>

scsi: Move core emulation state into a new struct

In preparation for sharing the emulation code between two drivers, move
some of the fields into a new struct. Use a separate header file so it
can be used by various drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>

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>

sandbox: scsi: Move file size into shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

sandbox: scsi: Move block size into shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

scsi: Move vendor/product info into the shared struct

Move this information into struct scsi_emul_info so we can use it in
common code.

Signed-off-by: Simon Glass <sjg@chromium.org>

sandbox: Move buffer to scsi_emul_info

Move the buffer into this struct so it can be shared between different
implementations.

Signed-off-by: Simon Glass <sjg@chromium.org>

scsi: Move core emulation state into a new struct

In preparation for sharing the emulation code between two drivers, move
some of the fields into a new struct. Use a separate header file so it
can be used by various drivers.

Signed-off-by: Simon Glass <sjg@chromium.org>