When setting up host notifiers, virtio_bus_set_host_notifier()
simply switches the handler. This will only work, however, if
the ioeventfd has already been setup; this is true for dataplane,
but not for vhost, and will completely break things if the
ioeventfd is disabled for the device.
Fix this by starting the ioeventfd on assign if that has not
happened before, and only switch the handler if the ioeventfd
has really been started.
While we're at it, also fixup the unsetting path of
set_host_notifier_internal().