filesystem: fix path property from Raid - override attribute

The following commmit:
  ce146ab28 add .path to Raid so for_client(raid-with-partitions) works

introduced a (read only) path property for Raid objects, returning an
imaginary value to make for_client work with Raid objects. Later on, the
following commit:
  8e658998e add "path" attributes to fs model objects that curtin now
            provides a path for

introduced a (read/write) path attribute to all filesystem objects. For
Raid however, the property still takes precedence over the new attribute
of the same name, so doing raid.path = x is invalid (no setter) and
results in the following exception:

AttributeError: can't set attribute 'path'

Fixed by using a @property + @setter attribute for path. The getter
returns the actual path if it exists, otherwise returns the same
imaginary value as before.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/models/filesystem.py

@@ -820,16 +820,22 @@ class Raid(_Device):
     wipe = attr.ib(default=None)
     ptable = attributes.ptable()
     metadata = attr.ib(default=None)
-    path = attr.ib(default=None)
+    _path = attr.ib(default=None)
     container = attributes.ref(backlink="_subvolumes", default=None)  # Raid
     _subvolumes = attributes.backlink(default=attr.Factory(list))
 
     @property
     def path(self):
+        if self._path is not None:
+            return self._path
         # This is just here to make for_client(raid-with-partitions) work. It
         # might not be very accurate.
         return '/dev/md/' + self.name
 
+    @path.setter
+    def path(self, value):
+        self._path = value
+
     @property
     def size(self):
         if self.preserve and self._m._probe_data: