migration: Document we don't copy storage during offline migration

https://bugzilla.redhat.com/show_bug.cgi?id=1322717

During offline migration, no storage is copied. Nor disks, nor
NVRAM file, nor anything. We use qemu for that and because domain
is not running there's nobody to copy that for us.
We should document this to avoid confusing users.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/migration.html.in

@@ -148,8 +148,9 @@
       <code>virsh dumpxml</code> on source host followed by
       <code>virsh define</code> on destination host, as offline migration
       will run the pre-migration hook to update the domain XML on
-      destination host. Currently, copying non-shared storage is not
-      supported during offline migration.
+      destination host. Currently, copying non-shared storage or other file
+      based storages (e.g. UEFI variable storage) is not supported during
+      offline migration.
     </p>
 
     <h2><a name="uris">Migration URIs</a></h2>

src/libvirt-domain.c

@@ -3574,6 +3574,10 @@ virDomainMigrateUnmanaged(virDomainPtr domain,
  * not support this feature and will return an error if bandwidth
  * is not 0.
  *
+ * Users should note that implementation of VIR_MIGRATE_OFFLINE
+ * flag in some drivers does not copy storage or any other file
+ * based storage (e.g. UEFI variable storage).
+ *
  * Enabling the VIR_MIGRATE_POSTCOPY flag tells libvirt to enable post-copy
  * migration.  Use virDomainMigrateStartPostCopy to switch migration into
  * the post-copy mode.  See virDomainMigrateStartPostCopy for more details
@@ -3791,6 +3795,10 @@ virDomainMigrate(virDomainPtr domain,
  * not support this feature and will return an error if bandwidth
  * is not 0.
  *
+ * Users should note that implementation of VIR_MIGRATE_OFFLINE
+ * flag in some drivers does not copy storage or any other file
+ * based storage (e.g. UEFI variable storage).
+ *
  * Enabling the VIR_MIGRATE_POSTCOPY flag tells libvirt to enable post-copy
  * migration.  Use virDomainMigrateStartPostCopy to switch migration into
  * the post-copy mode.  See virDomainMigrateStartPostCopy for more details
@@ -3980,6 +3988,10 @@ virDomainMigrate2(virDomainPtr domain,
  * can use either VIR_MIGRATE_NON_SHARED_DISK or
  * VIR_MIGRATE_NON_SHARED_INC as they are mutually exclusive.
  *
+ * Users should note that implementation of VIR_MIGRATE_OFFLINE
+ * flag in some drivers does not copy storage or any other file
+ * based storage (e.g. UEFI variable storage).
+ *
  * Enabling the VIR_MIGRATE_POSTCOPY flag tells libvirt to enable post-copy
  * migration.  Use virDomainMigrateStartPostCopy to switch migration into
  * the post-copy mode.  See virDomainMigrateStartPostCopy for more details