qemu: skip precreation of network disks

Commit cf54c606 introduced the ability
to create missing storage volumes during migration. For network disks,
however, we may not necessarily be able to detect whether they already
exist -- there is no straight-forward way to map the disk to a storage
volume, and even if there were it's possible no configured storage pool
actually contains the disk.

It is better to assume the network disk exists in this case, rather than
aborting the migration completely. If the volume really is missing, QEMU
will generate an appropriate error later in the migration.
Signed-off-by: NMichael Chapman <mike@very.puzzling.org>
(cherry picked from commit a1b18051)

src/qemu/qemu_migration.c

@@ -1507,9 +1507,13 @@ qemuMigrationPrecreateDisk(virConnectPtr conn,
             flags |= VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA;
         break;
 
+    case VIR_STORAGE_TYPE_NETWORK:
+        VIR_DEBUG("Skipping creation of network disk '%s'",
+                  disk->dst);
+        return 0;
+
     case VIR_STORAGE_TYPE_BLOCK:
     case VIR_STORAGE_TYPE_DIR:
-    case VIR_STORAGE_TYPE_NETWORK:
     case VIR_STORAGE_TYPE_NONE:
     case VIR_STORAGE_TYPE_LAST:
         virReportError(VIR_ERR_INTERNAL_ERROR,