node_memory: Expose the APIs to virsh

node_memory: Expose the APIs to virsh

New command node-memory-tune to get/set the node memory parameters,
only two parameters are allowed to set (pages_to_scan, and sleep_millisecs,
see documents in this patch for more details).

Example of node-memory-tune's output:

Shared memory:
	pages_to_scan   100
	sleep_millisecs 20
	pages_shared    0
	pages_sharing   0
	pages_unshared  0
	pages_volatile  0
	full_scans      0