We set the vm state as VM_POWERING_OFF when RTVM is trying to poweroff by itself.
We will check it when trying to pause vCPUs of RTVM. Only if vm state equal to
VM_POWERING_OFF, we take action to pause the vCPUs of RTVM. Otherwise, we will
reject the pause request.
Tracked-On: #2865
Signed-off-by: Kaige Fu <kaige.fu@intel.com>