Implement a node device backend using libudev

* configure.in: add new --with-udev, disabled by default, and requiring
  libudev > 145
* src/node_device/node_device_udev.c src/node_device/node_device_udev.h:
  the new node device backend
* src/node_device/node_device_linux_sysfs.c: moved node_device_hal_linux.c
  to a better file name
* src/conf/node_device_conf.c src/conf/node_device_conf.h: add a couple
  of fields in node device definitions, and an API to look them up,
  remove a couple of unused fields from previous patch.
* src/node_device/node_device_driver.c src/node_device/node_device_driver.h:
  plug the new driver
* po/POTFILES.in src/Makefile.am src/libvirt_private.syms: add the new
  files and symbols
* src/util/util.h src/util/util.c: add a new convenience macro
  virBuildPath and virBuildPathInternal() function

daemon/libvirtd.c

@@ -877,8 +877,7 @@ static struct qemud_server *qemudInitialize(void) {
 #ifdef WITH_STORAGE_DIR
     storageRegister();
 #endif
-#if defined(WITH_NODE_DEVICES) && \
-    (defined(HAVE_HAL) || defined(HAVE_DEVKIT))
+#if defined(WITH_NODE_DEVICES)
     nodedevRegister();
 #endif
     secretRegister();