qemu: raise an error when trying to use readonly ide disks

The IDE bus doesn't support readonly disks, so inform the user with an
error message instead of let qemu fail with a more obscure "Device
'ide-hd' could not be initialized" error message.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1112939Signed-off-by: NGiuseppe Scrivano <gscrivan@redhat.com>

src/qemu/qemu_command.c

@@ -3385,8 +3385,15 @@ qemuBuildDriveStr(virConnectPtr conn,
         disk->bus != VIR_DOMAIN_DISK_BUS_IDE)
         virBufferAddLit(&opt, ",boot=on");
     if (disk->readonly &&
-        virQEMUCapsGet(qemuCaps, QEMU_CAPS_DRIVE_READONLY))
+        virQEMUCapsGet(qemuCaps, QEMU_CAPS_DRIVE_READONLY)) {
+        if (disk->bus == VIR_DOMAIN_DISK_BUS_IDE &&
+            disk->device == VIR_DOMAIN_DISK_DEVICE_DISK) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("readonly ide disks are not supported"));
+            goto error;
+        }
         virBufferAddLit(&opt, ",readonly=on");
+    }
     if (disk->transient) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("transient disks not supported yet"));