virtio: move common irqfd handling out of virtio-pci

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