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>
