Switch SIG_IPI to SIGUSR1

Use SIGUSR1 unconditionally as SIG_IPI. First, ucontext coroutines tend
to corrupt RT signal masks due to a 32-on-64-bit Linux kernel bug. And,
second, there appears to be no advantage in using RT signals for VCPU
kicking.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

main-loop.h

@@ -25,11 +25,7 @@
 #ifndef QEMU_MAIN_LOOP_H
 #define QEMU_MAIN_LOOP_H 1
 
-#ifdef SIGRTMIN
-#define SIG_IPI (SIGRTMIN+4)
-#else
 #define SIG_IPI SIGUSR1
-#endif
 
 /**
  * qemu_init_main_loop: Set up the process so that it can run the main loop.