event_notifier: add event_notifier_set_handler

event_notifier: add event_notifier_set_handler

Win32 event notifiers are not file descriptors, so they will not be able
to use qemu_set_fd_handler.  But even if for now we only have a POSIX
version of EventNotifier, we can add a specific function that wraps
the call.

The wrapper passes the EventNotifier as the opaque value so that it will
be used with container_of.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>