When the <driver> element (and its "name" attribute) was added to the
domain XML's interface element, a "backend" enum was simply added to
the toplevel of the virDomainNetDef struct.

Ignoring the naming inconsistency ("name" vs. "backend"), this is fine
when there's only a single item contained in the driver element of the
XML, but doesn't scale well as we add more attributes that apply to
the backend of the virtio-net driver, or add attributes applicable to
other drivers.

This patch changes virDomainNetDef in two ways:

1) Rename the item in the struct from "backend" to "name", so that
   it's the same in the XML and in the struct, hopefully avoiding
   confusion for someone unfamiliar with the function of the
   attribute.

2) Create a "driver" union within virDomainNetDef, and a "virtio"
   struct in that struct, which contains the "name" enum value.

3) Move around the virDomainNetParse and virDomainNetFormat functions
   to allow for simple plugin of new attributes without disturbing
   existing code. (you'll note that this results in a seemingly
   redundant if() in the format function, but that will no longer be
   the case as soon as a 2nd attribute is added).

In the future, new attributes for the virtio driver backend can be
added to the "virtio" struct, and any other network device backend that
needs an attribute will have its own struct added to the "driver"
union.