storage/lvm: check for LVM LV existence and type when creating ThinPool

Check if requested thin pool exists and really is thin pool.

QubesOS/qubes-issues#3438

qubes/storage/lvm.py

@@ -98,6 +98,13 @@ class ThinPool(qubes.storage.Pool):
 
     def setup(self):
         reset_cache()
+        cache_key = self.volume_group + '/' + self.thin_pool
+        if cache_key not in size_cache:
+            raise qubes.storage.StoragePoolException(
+                'Thin pool {} does not exist'.format(cache_key))
+        if size_cache[cache_key]['attr'][0] != 't':
+            raise qubes.storage.StoragePoolException(
+                'Volume {} is not a thin pool'.format(cache_key))
         # TODO Should we create a non existing pool?
 
     def get_volume(self, vid):