All transports can use the same event handler for the irqfd, though the
exact mechanics of the assignment will be specific.  Note that there
are three states: handled by the kernel, handled in userspace, disabled.

This also lets virtio use event_notifier_set_handler.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>