libxl: restore passing "readonly=" to qemu for SCSI disks

A read-only check was introduced for XSA-142, commit ef6cb76026 ("libxl:
relax readonly check introduced by XSA-142 fix") added the passing of
the extra setting, but commit dab0539568 ("Introduce COLO mode and
refactor relevant function") dropped the passing of the setting again,
quite likely due to improper re-basing.

Restore the readonly= parameter to SCSI disks.  For IDE disks this is
supposed to be rejected; add an assert.  And there is a bare ad-hoc
disk drive string in libxl__build_device_model_args_new, which we also
update.

This is XSA-266.

Reported-by: Andrew Reimers <andrew.reimers@orionvm.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

tools/libxl/libxl_dm.c

@@ -798,7 +798,8 @@ static char *qemu_disk_scsi_drive_string(libxl__gc *gc, const char *target_path,
                                          int colo_mode, const char **id_ptr)
 {
     char *drive = NULL;
-    char *common = GCSPRINTF("if=none,cache=writeback");
+    char *common = GCSPRINTF("if=none,readonly=%s,cache=writeback",
+                             disk->readwrite ? "off" : "on");
     const char *exportname = disk->colo_export;
     const char *active_disk = disk->active_disk;
     const char *hidden_disk = disk->hidden_disk;
@@ -852,6 +853,8 @@ static char *qemu_disk_ide_drive_string(libxl__gc *gc, const char *target_path,
     const char *active_disk = disk->active_disk;
     const char *hidden_disk = disk->hidden_disk;
 
+    assert(disk->readwrite); /* should have been checked earlier */
+
     switch (colo_mode) {
     case LIBXL__COLO_NONE:
         drive = GCSPRINTF
@@ -1574,8 +1577,9 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
                     const char *drive_id;
                     if (colo_mode == LIBXL__COLO_SECONDARY) {
                         drive = libxl__sprintf
-                            (gc, "if=none,driver=%s,file=%s,id=%s",
-                             format, target_path, disks[i].colo_export);
+                            (gc, "if=none,driver=%s,file=%s,id=%s,readonly=%s",
+                             format, target_path, disks[i].colo_export,
+                             disks[i].readwrite ? "off" : "on");
 
                         flexarray_append(dm_args, "-drive");
                         flexarray_append(dm_args, drive);