Refactor cgroups to allow a group per driver to be managed directly

Refactor cgroups to allow a group per driver to be managed directly

Allow the driver level cgroup to be managed explicitly by the
hypervisor drivers, in order to detect whether to enable or
disable cgroup support for domains. Provides better error
reporting of failures. Also allow for creation of cgroups for
unprivileged drivers if controller is accessible by the user.

* src/cgroup.c, src/cgroup.h: Add an API to obtain a driver cgroup
* src/lxc_conf.h, src/lxc_controller.c, src/lxc_driver.c:
  Obtain a driver cgroup at startup and use that instead of
  re-creating everytime.
* src/util.c, src/util.h, src/libvirt_private.syms: Add a
  virGetUserName() helper