cpus: ignore ESRCH in qemu_cpu_kick_thread()

cpus: ignore ESRCH in qemu_cpu_kick_thread()

We can have a race condition between qemu_cpu_kick_thread() and
qemu_kvm_cpu_thread_fn() when we hotunplug a CPU. In this case,
qemu_cpu_kick_thread() can try to kick a thread that is exiting.
pthread_kill() returns an error and qemu is stopped by an exit(1).

   qemu:qemu_cpu_kick_thread: No such process

We can ignore safely this error.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>