reenable vm_clock when resuming all vcpus

We disable vm_clock when pausing all vcpus, but we forget to
reenable it when resuming all vcpus. It will cause that the
guest can not be rebooted.
Tested-by: NZhi Yong Wu <zwu.kernel@gmai.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

cpus.c

@@ -891,6 +891,7 @@ void resume_all_vcpus(void)
 {
     CPUState *penv = first_cpu;
 
+    qemu_clock_enable(vm_clock, true);
     while (penv) {
         penv->stop = 0;
         penv->stopped = 0;