Instance is already powered off in the hypervisor when stop is called
Nova live-migration failing for RHEL7CentOS7 VMs
主要原因是因为以下3个软件包版本的bug导致（一旦从一个计算节点移到另外一个计算节点，就出先电源状态为"关闭"的情况出现）：
qemu-kvm-1.5.3-105.el72.7 qemu-img-1.5.3-105.el72.7 qemu-kvm-common-1.5.3-105.el7_2.7
解决方法：在相应计算节点把qemu-kvm、qemu-img和qemu-kvm-common的版本降为2.4即可。
yum downgrade qemu-kvm qemu-kvm-common qemu-img
例如：
1.从其他节点迁移到节点11时是成功的：
root@compute11 nova]# yum list installed | grep "qemu-" qemu-common.x86_64 2:2.0.0-1.el7.6 @epel qemu-img.x86_64 10:1.5.3-105.el7_2.4 @updates qemu-kvm.x86_64 10:1.5.3-105.el7_2.4 @updates qemu-kvm-common.x86_64 10:1.5.3-105.el7_2.4 @updates
2.从其他节点迁移到节点11时是失败的：
[root@compute20 qemu]# yum list installed | grep "qemu-" qemu-common.x86_64 2:2.0.0-1.el7.6 @epel qemu-guest-agent.x86_64 10:2.3.0-4.el7 @base qemu-img.x86_64 10:1.5.3-105.el7_2.7 @updates qemu-kvm.x86_64 10:1.5.3-105.el7_2.7 @updates qemu-kvm-common.x86_64 10:1.5.3-105.el7_2.7 @updates qemu-kvm-tools.x86_64 10:1.5.3-105.el7_2.7 @updates
解决方法：
yum downgrade qemu-kvm qemu-kvm-common qemu-img