interface: add udev based backend for virInterface

interface: add udev based backend for virInterface

Add a read-only udev based backend for virInterface. Useful for distros
that do not have netcf support yet. Multiple libvirt based utilities use
a HAL based fallback when virInterface is not available which is less
than ideal. This implements:
* virConnectNumOfInterfaces()
* virConnectListInterfaces()
* virConnectNumOfDefinedInterfaces()
* virConnectListDefinedInterfaces()
* virConnectListAllInterfaces()
* virConnectInterfaceLookupByName()
* virConnectInterfaceLookupByMACString()