module/cgroups: Really move all tasks in Controller.move_all_tasks_to()

The docstring of Controller.move_all_tasks_to() says that the function
moves all the tasks to the "dest" cgroup.  However, it iterates over
self._cgroups, which is a dictionary that is lazily populated when you
call Controller.cgroup().  For example, this doesn't work:

cpuset_cg = target.cgroups.controller("cpuset")
cpuset_cg.move_all_tasks_to("top-app")

Because you haven't populated self._cgroups yet.  You need to manually
populate the dictionary with something like:

for group in cpuset_cg.list_all():
    cpuset_cg.cgroup(group)

before you can use move_all_tasks_to().  Iterate through
self.list_all() instead of self._cgroups to really move all tasks to
to the destination directory.

Controller.move_tasks() has a try-except block to get the cgroups of
the source and destination groups.  Controller.cgroup() caches the
groups in self._cgroups and populates it if it hasn't been already.
Simplify move_tasks() and let it deal with source and dest cgroups
that exist but the controller hasn't loaded yet.

devlib/module/cgroups.py

@@ -124,11 +124,10 @@ class Controller(object):
     def move_tasks(self, source, dest, exclude=None):
         if exclude is None:
             exclude = []
-        try:
-            srcg = self._cgroups[source]
-            dstg = self._cgroups[dest]
-        except KeyError as e:
-            raise ValueError('Unknown group: {}'.format(e))
+
+        srcg = self.cgroup(source)
+        dstg = self.cgroup(dest)
+
         self.target._execute_util(  # pylint: disable=protected-access
                     'cgroups_tasks_move {} {} \'{}\''.format(
                     srcg.directory, dstg.directory, exclude),
@@ -169,7 +168,7 @@ class Controller(object):
             self.logger.debug('   excluding tasks which name matches:')
             self.logger.debug('   %s', ', '.join(exclude))
 
-        for cgroup in self._cgroups:
+        for cgroup in self.list_all():
             if cgroup != dest:
                 self.move_tasks(cgroup, dest, grep_filters)