libvirt获取实际内存的实现
libvirt可以通过virDomainGetInfo()来得到虚机信息：
struct virDomainInfo{
unsigned char state : the running state, one of virDomainState
unsigned long maxMem : the maximum memory in KBytes allowed
unsigned long memory : the memory in KBytes used by the domain
unsigned short nrVirtCpu : the number of virtual CPUs for the domain
unsigned long long cpuTime : the CPU time used in nanoseconds
}
libvirt获取虚拟机实际内存的实现方法：
1. 通过libxl_domain_info()：provides a way to get the struct libxl_dominfo
for a single domain given its domid.
代码里是读取struct libxl_dominfo -> current_memkb
来得到实际内存，但是从网上搜到的结构体为：
struct libxl_dominfo {
uint8_t uuid[16];
uint32_t domid;
uint8_t dying:1;
uint8_t paused:1;
uint8_t running:1;
uint64_t max_memkb;
uint64_t cpu_time;
uint32_t vcpu_max_id;
uint32_t vcpu_online;
};
xen-unstable提供了这个接口
2. 通过xenapi: xen_vm_get_record() xen_vm_metrics_get_memory_actual()
3. QEMU: 通过BallonInfo来获取内存
