If we call qemu_cpu_kick more than once before the target was able to
process the signal, pthread_kill will fail, and qemu will abort. Prevent
this by avoiding the redundant signal.

This logic can be found in qemu-kvm as well.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>