osbuild: error when {Device,Mount} is modified after creation

This is a drive-by change after spending some quality time with the
mount code. The `id` field of `Mount` is calculated only once and
only when creating a `Mount`. This seems slightly dangerous as
any change to an attribute after creation will not update the
id. This means two options:
1. dynamically update the `id` on changes
2. forbid changes after the `id` is calculcated

I went with (2) but happy to discuss of course but it seems more
the spirit of the class.

It also does the same change for "devices.Device"

osbuild/mounts.py

@@ -16,9 +16,10 @@ from typing import Dict, List
 
 from osbuild import host
 from osbuild.devices import DeviceManager
+from osbuild.mixins import MixinImmutableID
 
 
-class Mount:
+class Mount(MixinImmutableID):
     """
     A single mount with its corresponding options
     """