Assure multiprocess start method is fork

Python 3.14 in Fedora 43 changes the default start method for
multiprocessing away from fork. We set it back to `fork`.

resolves: QubesOS/qubes-issues#10225

Author: alimirjamali

qubesadmin/backup/restore.py

@@ -55,6 +55,10 @@
 from qubesadmin.utils import size_to_human
 
 
+# Python 3.14 in Fedora 43 changes the default start method away from fork
+if multiprocessing.get_start_method(allow_none=True) != "fork":
+    multiprocessing.set_start_method("fork")
+
 # must be picklable
 QUEUE_FINISHED = "!!!FINISHED"
 QUEUE_ERROR = "!!!ERROR"