net: Flatten simple union NetLegacyOptions

Simple unions are simpler than flat unions in the schema, but more
complicated in C and on the QMP wire: there's extra indirection in C
and extra nesting on the wire, both pointless.  They're best avoided
in new code.

NetLegacyOptions isn't new, but it's only used internally, not in QMP.
Convert it to a flat union.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487709988-14322-3-git-send-email-armbru@redhat.com>