virtio: move common ioeventfd handling out of virtio-pci

All transports can use the same event handler for the ioeventfd, though
the exact setup (address/memory region) will be specific.

This lets virtio use event_notifier_set_handler.

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