scsi-generic: avoid out-of-bounds access to VPD page list

A device can report an excessive number of VPD pages when asked for a
list; this can cause an out-of-bounds access to buf in
scsi_generic_set_vpd_bl_emulation.  It should not happen, but
it is technically not incorrect so handle it: do not check any byte
past the allocation length that was sent to the INQUIRY command.
Reported-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-generic.c

@@ -538,7 +538,7 @@ static void scsi_generic_set_vpd_bl_emulation(SCSIDevice *s)
     }
 
     page_len = buf[3];
-    for (i = 4; i < page_len + 4; i++) {
+    for (i = 4; i < MIN(sizeof(buf), page_len + 4); i++) {
         if (buf[i] == 0xb0) {
             s->needs_vpd_bl_emulation = false;
             return;