Currently building --without-libvirtd causes a failure to link the node
device driver:

node_device/.libs/libvirt_driver_nodedev_la-node_device_driver.o: In function `nodedevRegister':
/home/berrange/src/virt/libvirt/src/node_device/node_device_driver.c:649: undefined reference to `udevNodeRegister'
collect2: error: ld returned 1 exit status

because it causes us to build the core nodedev driver, but then skip the
implementations, despite udev being available.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>