fix qemu-kvm sigsegv at exit

[Marcelo Tosatti <mtosatti@redhat.com>]

Michael reported a qemu-kvm SIGSEGV at shutdown:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x411d0940 (LWP 14446)]
0x000000000040afb4 in qemu_mod_timer (ts=0x19f0fd0,
expire_time=62275467335)
    at /home/mst/scm/qemu-kvm/vl.c:1009
1009            if ((alarm_timer->flags & ALARM_FLAG_EXPIRED) == 0)
{
(gdb) l
1004        ts->next = *pt;
1005        *pt = ts;
1006
1007        /* Rearm if necessary  */
1008        if (pt == &active_timers[ts->clock->type]) {
1009            if ((alarm_timer->flags & ALARM_FLAG_EXPIRED) == 0)
{
1010          	qemu_rearm_alarm_timer(alarm_timer);
1011            }
1012            /* Interrupt execution to force deadline
recalculation.  */
1013            if (use_icount)
(gdb) p alarm_timer
$1 = (struct qemu_alarm_timer *) 0x0

Problem is kvm_main_loop_wait(env, 0) can process the stop request
(signalling iothread that vcpu is stopped, so its OK to exit) and
continue to kvm_cpu_exec.

Make sure cpu is not stopped before proceeding to kvm_cpu_exec.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reported-by: "Michael S. Tsirkin" <mst@redhat.com>

diff --git a/qemu-kvm.c b/qemu-kvm.c
index 4c13628..ab8f0e4 100644
--- a/qemu-kvm.c
+++ b/qemu-kvm.c
@@ -1868,7 +1868,8 @@ static int kvm_main_loop_cpu(CPUState *env)
         }
         if (run_cpu) {
             kvm_main_loop_wait(env, 0);
-            kvm_cpu_exec(env);
+            if (!is_cpu_stopped(env))
+                kvm_cpu_exec(env);
         } else {
             kvm_main_loop_wait(env, 1000);
         }