This is identical to type='bridge', but without the "connect to a
bridge" part, so it can be handled by using the same functions (and
often even the same cases in switch statements), after renaming
virLXCProcessSetupInterfaceBridged() to virLXCProcessInterfaceTap()
and enhancing it to skip bridge-related items when brname == NULL.

To be truly useful, we need to support setting the ip address on the
host side veth as well as guest side veth (already supported for
type='bridge'), as well as setting the peer address for both.

The <script> element (supported by type='ethernet' in qemu) isn't
supported in this patch. An error is logged at domain start time if it
is encountered. This may be changed in a later patch.