Use the UUID for the machine ID

This avoids leaking the real machine ID into guests, and provides an
easy way for guests to know their own UUID.

Fixes: QubesOS/qubes-issues#8833

qubes/vm/qubesvm.py

@@ -2533,16 +2533,17 @@ def kernelopts_common(self):
         """
         if not self.kernel:
             return ""
+        base_kernelopts = "systemd.machine_id=" + self.uuid.hex + " "
         kernels_dir = self.storage.kernels_dir
 
         kernelopts_path = os.path.join(
             kernels_dir, "default-kernelopts-common.txt"
         )
         if os.path.exists(kernelopts_path):
             with open(kernelopts_path, encoding="ascii") as f_kernelopts:
-                return f_kernelopts.read().rstrip("\n\r")
+                return base_kernelopts + f_kernelopts.read().rstrip("\n\r")
         else:
-            return qubes.config.defaults["kernelopts_common"]
+            return base_kernelopts + qubes.config.defaults["kernelopts_common"]
 
     #
     # helper methods