libvirtInterface: Convert some keys of mem_stats in getVmMemoryStats()

libvirtInterface.mem_stats is a dictionary to convert keys of mem_stats
from libvirt internal name to that of used in mom rules, but its value was
not actually used. This patch fixes this.

Change-Id: I9281a332419f4a7a52764e4def265d742080252c
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-on: http://gerrit.ovirt.org/19198
Reviewed-by: Adam Litke <agl@us.ibm.com>
Tested-by: Adam Litke <agl@us.ibm.com>