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"

test/mod/test_mixins.py

@@ -0,0 +1,32 @@
+from unittest.mock import Mock
+
+import pytest
+
+from osbuild.devices import Device
+from osbuild.meta import ModuleInfo
+from osbuild.mounts import Mount
+
+
+def test_mount_immutable_mixin():
+    info = Mock(spec=ModuleInfo)
+    info.name = "some-name"
+    device = Mock(spec=ModuleInfo)
+    device.id = "some-id"
+    partition = 1
+    target = "/"
+    opts = {"opt1": 1}
+    mnt = Mount("name", info, device, partition, target, opts)
+    with pytest.raises(ValueError) as e:
+        mnt.name = "new-name"
+    assert str(e.value) == "cannot set 'name': Mount cannot be changed after creation"
+
+
+def test_device_immutable_mixins():
+    info = Mock(spec=ModuleInfo)
+    info.name = "some-name"
+    parent = None
+    opts = {"opt1": 1}
+    dev = Device("name", info, parent, opts)
+    with pytest.raises(ValueError) as e:
+        dev.name = "new-name"
+    assert str(e.value) == "cannot set 'name': Device cannot be changed after creation"