This was accomplished in xml parsing by doing away with the
stripped-down virInterfaceBareDef object, and just always using
virInterfaceDef, but with restrictions in certain places (eg, the type
of subordinate interface allowed in parsing depends on the parent
interface).

xml formatting was similarly adjusted. In addition, the formatting
functions keep track of the level of interface nesting, and insert
extra leading spaces on each line accordingly (using %*s).

The only change in formatted xml from previous (aside frmo supporting
new combinations of interface types) is that the subordinate ethernet
interfaces take up 2 lines rather than one, eg:

   <interface type='ethernet' name='eth0'>
   </interface>

instead of:

   <interface type='ethernet' name='eth0'/>