tweak qemu_notify_event

Instead of testing specially next_cpu in host_alarm_handler, just do
that in qemu_notify_event.  The idea is, if we are not running (or
not yet running) target CPU code, prepare things so that the execution
loop is exited asap; just make that clear.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
