util: eliminate union in virNetDevVPortProfile

util: eliminate union in virNetDevVPortProfile

virNetDevVPortProfile has (had) a type field that can be set to one of
several values, and a union of several structs, one for each
type. When a domain's interface object is of type "network", the
domain config may not know beforehand which type of virtualport is
going to be provided in the actual device handed down from the network
driver at runtime, but may want to set some values in the virtualport
that may or may not be used, depending on the type. To support this
usage, this patch replaces the union of structs with toplevel fields
in the struct, making it possible for all of the fields to be set at
the same time.