Hello,
I think it is abnormal when the process of qemu-kvm does not release unused resident memory .
For example, at start the virtual server qemu-kvm uses memory as: VIRT = 2322m, RES = 660m
I fill his memory running the script inside virtual server, but when memory is freed inside the container,  RES= 2.0g and never had not diminished.

So it must be, or this can be changed?

kernel: 2.6.32-71.29.1.el6.x86_64
libvirt-0.8.1-27.el6_0.6.x86_64
qemu-kvm-0.12.1.2-2.113.el6_0.8.x86_64
qemu-kvm-tools-0.12.1.2-2.113.el6_0.8.x86_64

script for filling memory 

#!/usr/bin/perl
use strict;
$|=1;
my @a;
my $allocated=0;
my $blocksize=10;
foreach my $i(1..1024){
  my $time = time();
  my $s = '1' x ($blocksize*1024*1024);
  push @a, $s;
  $allocated += $blocksize;
  print "\r$allocated Mb";
  $s = $a[$i-10]; # read data - from swap slow!
  if( time()-$time > 10 ){
    print "\n$allocated Mb: Slow memory allocation\n";
    last;
  }