managedsave: prohibit use on transient domains

Transient domains reject attempts to set autostart, and using
virDomainCreate to restart a domain only works on persistent
domains.  Therefore, managed save makes no sense on transient
domains, and should be rejected up front rather than creating
an otherwise unrecoverable managed save file.

Besides, transient domains imply that a lot more management is
being done by the upper layer; this includes the assumption
that the upper layer is okay managing the saved state file
created by virDomainSave, and does not need to use managed save.

* src/libvirt.c: Document that transient domains are incompatible
with managed save.
* src/qemu/qemu_driver.c (qemuDomainManagedSave): Enforce it.
* src/libxl/libxl_driver.c (libxlDomainManagedSave): Likewise.

src/libvirt.c

@@ -15285,6 +15285,9 @@ error:
  * the saved state itself, and will reuse it once the domain is being
  * restarted (automatically or via an explicit libvirt call).
  * As a result any running domain is sure to not have a managed saved image.
+ * This also implies that managed save only works on persistent domains,
+ * since the domain must still exist in order to use virDomainCreate() to
+ * restart it.
  *
  * If @flags includes VIR_DOMAIN_SAVE_BYPASS_CACHE, then libvirt will
  * attempt to bypass the file system cache while creating the file, or

src/libxl/libxl_driver.c

@@ -2132,6 +2132,11 @@ libxlDomainManagedSave(virDomainPtr dom, unsigned int flags)
         libxlError(VIR_ERR_OPERATION_INVALID, "%s", _("Domain is not running"));
         goto cleanup;
     }
+    if (!vm->persistent) {
+        libxlError(VIR_ERR_OPERATION_INVALID, "%s",
+                   _("cannot do managed save for transient domain"));
+        goto cleanup;
+    }
 
     name = libxlDomainManagedSavePath(driver, vm);
     if (name == NULL)

src/qemu/qemu_driver.c

@@ -2582,6 +2582,11 @@ qemuDomainManagedSave(virDomainPtr dom, unsigned int flags)
                         "%s", _("domain is not running"));
         goto cleanup;
     }
+    if (!vm->persistent) {
+        qemuReportError(VIR_ERR_OPERATION_INVALID, "%s",
+                        _("cannot do managed save for transient domain"));
+        goto cleanup;
+    }
 
     name = qemuDomainManagedSavePath(driver, vm);
     if (name == NULL)