1. for guest xml desc:
<memballoon model='virtio'/>
一定要设置<currentMemory>，这个bollon没有起作用
2. 在windows guest里面安装ballooning驱动：
a) 下载并安装devcon, http://support.microsoft.com/kb/311272
b) devcon install BALLOON.inf "PCI\VEN_1AF4&DEV_1002&SUBSYS_00051AF4&REV_00"
3. 重启guest, 使用qemu monitor测试:
virsh # qemu-monitor-command winxp --hmp "info balloon"
balloon: actual=1024
virsh # qemu-monitor-command winxp --hmp "balloon 512"
virsh # qemu-monitor-command winxp --hmp "info balloon"
balloon: actual=512