Move parents checking and update into a seprarate methods

Internal LVs are not going to be referenced by the DeviceTree instance and are
going to have no parents. Their "parent" LVs are going to be referenced as
'self.parent_lv' instead of 'self.parents[0]' because all the other devices use
the opposite logic for parent-children relations -- children are consisting of
or built on top of parents (whereas "parent" LVs are consisting of their
internal LVs).

This way internal LVs can easily override these methods.

blivet/devices/lvm.py

@@ -489,16 +489,6 @@ def __init__(self, name, parents=None, size=None, uuid=None,
             :type percent: int
 
         """
-        if isinstance(parents, list):
-            if len(parents) != 1:
-                raise ValueError("constructor requires a single %s instance" % self._containerClass.__name__)
-
-            container = parents[0]
-        else:
-            container = parents
-
-        if not isinstance(container, self._containerClass):
-            raise ValueError("constructor requires a %s instance" % self._containerClass.__name__)
 
         DMDevice.__init__(self, name, size=size, fmt=fmt,
                           sysfsPath=sysfsPath, parents=parents,
@@ -523,8 +513,29 @@ def __init__(self, name, parents=None, size=None, uuid=None,
             self.req_size = self._size
             self.req_percent = util.numeric_type(percent)
 
-        # here we go with the circular references
-        self.parents[0]._addLogVol(self)
+        # check that we got parents as expected and add this device to them
+        self._check_parents()
+        self._add_to_parents()
+
+    def _check_parents(self):
+        """Check that this device has parents as expected"""
+
+        if isinstance(self.parents, list):
+            if len(self.parents) != 1:
+                raise ValueError("constructor requires a single %s instance" % self._containerClass.__name__)
+
+            container = self.parents[0]
+        else:
+            container = self.parents
+
+        if not isinstance(container, self._containerClass):
+            raise ValueError("constructor requires a %s instance" % self._containerClass.__name__)
+
+    def _add_to_parents(self):
+        """Add this device to its parents"""
+
+        # a normal LV has only exactly parent -- the VG it belongs to
+        self._parents[0]._addLogVol(self)
 
     def __repr__(self):
         s = DMDevice.__repr__(self)