qemu: Perform the disk WWN check only on fresh starts

Since we'd disallow migration of a guest that would have possibly
invalid config but still be able to work, relax the WWN check to be
performed only on new starts of the VM.

src/qemu/qemu_process.c

@@ -4628,7 +4628,8 @@ int qemuProcessStart(virConnectPtr conn,
             goto cleanup;
     }
 
-    if (virDomainDefCheckDuplicateDiskWWN(vm->def) < 0)
+    if (!migrateFrom && !snapshot &&
+        virDomainDefCheckDuplicateDiskWWN(vm->def) < 0)
         goto cleanup;
 
     /* "volume" type disk's source must be translated before