Revert "libxl: implement virDomainInterfaceStats"

Revert "libxl: implement virDomainInterfaceStats"

This reverts commit d2e5538b16e325d9095f3ccb0dac88bbd9fc98f0.

A migration regression was introduced by this commit. When migrating
a domain, its active XML is sent to the destination libvirtd, where
it is parsed as inactive XML. d2e5538b copied the libxl generated
interface name into the active config, which was being passed to the
migration destination and being parsed into inactive config. Attempting
to start the config could result in failure if an interface with the
same generated name already exists.

The qemu driver behaves similarly, but the parser contains a hack to
skip interface names starting with 'vnet' when parsing inactive XML.
We could extend the hack to skip names starting with 'vif' too, but a
better fix would be to expose these hypervisor-specific interface name
prefixes in capabilities. See the following discussion thread for more
details

https://www.redhat.com/archives/libvir-list/2015-December/msg00262.html

For the pending 1.3.0 release, it is best to revert d2e5538b. It can
be added again post release, after moving the prefix to capabilities.