HV: Set vm state as with VM_POWERING_OFF when RTVM poweroff by itself

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>

hypervisor/include/arch/x86/guest/vm.h

@@ -88,6 +88,7 @@ enum vm_state {
 	VM_STATE_INVALID = 0,
 	VM_CREATED,	/* VM created / awaiting start (boot) */
 	VM_STARTED,	/* VM started (booted) */
+	VM_POWERING_OFF,     /* RTVM only, it is trying to poweroff by itself */
 	VM_PAUSED,	/* VM paused */
 };