libxl: handle read-only drives with qemu-xen

The current libxl code doesn't deal with read-only drives at all.

Upstream QEMU and qemu-xen only support read-only cdrom drives: make
sure to specify "readonly=on" for cdrom drives and return error in case
the user requested a non-cdrom read-only drive.

This is XSA-142, discovered by Lin Liu
(https://bugzilla.redhat.com/show_bug.cgi?id=1257893).

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_dm.c

@@ -1110,13 +1110,18 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
             if (disks[i].is_cdrom) {
                 if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY)
                     drive = libxl__sprintf
-                        (gc, "if=ide,index=%d,media=cdrom,cache=writeback,id=ide-%i",
-                         disk, dev_number);
+                        (gc, "if=ide,index=%d,readonly=%s,media=cdrom,cache=writeback,id=ide-%i",
+                         disk, disks[i].readwrite ? "off" : "on", dev_number);
                 else
                     drive = libxl__sprintf
-                        (gc, "file=%s,if=ide,index=%d,media=cdrom,format=%s,cache=writeback,id=ide-%i",
-                         disks[i].pdev_path, disk, format, dev_number);
+                        (gc, "file=%s,if=ide,index=%d,readonly=%s,media=cdrom,format=%s,cache=writeback,id=ide-%i",
+                         disks[i].pdev_path, disk, disks[i].readwrite ? "off" : "on", format, dev_number);
             } else {
+                if (!disks[i].readwrite) {
+                    LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "qemu-xen doesn't support read-only disk drivers");
+                    return ERROR_INVAL;
+                }
+
                 if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY) {
                     LIBXL__LOG(ctx, LIBXL__LOG_WARNING, "cannot support"
                                " empty disk format for %s", disks[i].vdev);