Teardown all descendants devices before tearing disk images down

DM maps cannot be removed while they are in use so in order to make them
removable, we need to teardown all devices "built" on top of them first. This
makes our examples (and possibly some tests) better clean after themselves.

blivet/devicetree.py

@@ -1048,6 +1048,11 @@ def setup_disk_images(self):
 
     def teardown_disk_images(self):
         """ Tear down any disk image stacks. """
+        for dev_name in self.disk_images.keys():
+            dm_device = self.get_device_by_name(dev_name)
+            descendants = self.get_dependent_devices(dm_device)
+            for dev in reversed(descendants):
+                dev.teardown(recursive=True)
         self._populator.teardown_disk_images()
 
     @property