snapshot: reject transient disks where code is not ready

The previous patch introduced new config, but if a hypervisor does
not support that new config, someone can write XML that does not
behave as documented.  This prevents some of those cases by
explicitly rejecting transient disks for several hypervisors.

Disk snapshots will require a new flag to actually affect a snapshot
creation, so there's not much to reject there.

* src/qemu/qemu_command.c (qemuBuildDriveStr): Reject transient
disks for now.
* src/libxl/libxl_conf.c (libxlMakeDisk): Likewise.
* src/xenxs/xen_sxpr.c (xenFormatSxprDisk): Likewise.
* src/xenxs/xen_xm.c (xenFormatXMDisk): Likewise.

src/libxl/libxl_conf.c

@@ -537,6 +537,11 @@ libxlMakeDisk(virDomainDefPtr def, virDomainDiskDefPtr l_disk,
     x_disk->unpluggable = 1;
     x_disk->readwrite = !l_disk->readonly;
     x_disk->is_cdrom = l_disk->device == VIR_DOMAIN_DISK_DEVICE_CDROM ? 1 : 0;
+    if (l_disk->transient) {
+        libxlError(VIR_ERR_INTERNAL_ERROR, "%s",
+                   _("libxenlight does not support transient disks"));
+        return -1;
+    }
 
     x_disk->domid = def->id;
 

src/qemu/qemu_command.c

@@ -1594,6 +1594,11 @@ qemuBuildDriveStr(virDomainDiskDefPtr disk,
     if (disk->readonly &&
         qemuCapsGet(qemuCaps, QEMU_CAPS_DRIVE_READONLY))
         virBufferAddLit(&opt, ",readonly=on");
+    if (disk->transient) {
+        qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                        _("transient disks not supported yet"));
+        goto error;
+    }
     if (disk->driverType && *disk->driverType != '\0' &&
         disk->type != VIR_DOMAIN_DISK_TYPE_DIR &&
         qemuCapsGet(qemuCaps, QEMU_CAPS_DRIVE_FORMAT))

src/xenxs/xen_sxpr.c

@@ -1713,6 +1713,11 @@ xenFormatSxprDisk(virDomainDiskDefPtr def,
         virBufferAddLit(buf, "(mode 'w!')");
     else
         virBufferAddLit(buf, "(mode 'w')");
+    if (def->transient) {
+        XENXS_ERROR(VIR_ERR_CONFIG_UNSUPPORTED,
+                    _("transient disks not supported yet"));
+        return -1;
+    }
 
     if (!isAttach)
         virBufferAddLit(buf, ")");

src/xenxs/xen_xm.c

@@ -1164,6 +1164,11 @@ static int xenFormatXMDisk(virConfValuePtr list,
         virBufferAddLit(&buf, ",!");
     else
         virBufferAddLit(&buf, ",w");
+    if (disk->transient) {
+        XENXS_ERROR(VIR_ERR_CONFIG_UNSUPPORTED,
+                    _("transient disks not supported yet"));
+        return -1;
+    }
 
     if (virBufferError(&buf)) {
         virReportOOMError();