The patch implements the missing memory control APIs for lxc, i.e.,
domainGetMaxMemory, domainSetMaxMemory, domainSetMemory, and improves
domainGetInfo to return proper amount of used memory via cgroup.

* src/libvirt_private.syms: Export virCgroupGetMemoryUsage
  and add missing virCgroupSetMemory
* src/lxc/lxc_driver.c: Implement missing memory functions
* src/util/cgroup.c, src/util/cgroup.h: Add the function
  to get used memory