coroutine: Fix setup of sigaltstack coroutines

Use pthread_kill instead of process-wide kill to invoke the signal
handler used for stack switching. This may fix spurious lock-ups with
this backend, easily triggerable by extending the time window between
kill and sigsuspend.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

coroutine-sigaltstack.c

@@ -226,7 +226,7 @@ static Coroutine *coroutine_new(void)
      * called.
      */
     coTS->tr_called = 0;
-    kill(getpid(), SIGUSR2);
+    pthread_kill(pthread_self(), SIGUSR2);
     sigfillset(&sigs);
     sigdelset(&sigs, SIGUSR2);
     while (!coTS->tr_called) {