scsi: xen-scsifront: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases where
we are expecting to fall through.

Notice that, in this particular case, I replaced "Missed the backend's
Closing state -- fallthrough" with "fall through - Missed the backend's
Closing state", which contains the "fall through" annotation at the
beginnig of the code comment, which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>