LXC: Include cputime in return value of GetInstanceInfo

Read the cputime(usage) of the LXC instance from the cpuacct.usage
cgroup parameter and return it as the "time" field of GetInstanceInfo.
This commit also discards the TODO comment wrt something which is not
accurate today.
LXC: Remove comment

This commit discards comments in LXCHypervisor which are already
solved, or not accurate today.
LXC: Remove unused function

Remove the unused _GetMountSubdirs function from LXCHypervisor class.
LXC: Add lxc_extra_config hvparam

Add the new hvparam lxc_extra_config which is used to specify additional
LXC container config parameters that is not supported by the Ganeti LXC
hypervisor natively.
LXC: Raise an error if failed to read cgroup memory limit

Since we have supported the memory limit for the LXC container, we
should always have the memory subsystem mounted somewere, so getting
memory limit from cgroup fs is not an acceptable situation.