conf: eliminate outmoded/odd indent method from interface xml

conf: eliminate outmoded/odd indent method from interface xml

These format functions needed the ability to be indented by an
arbitrary amount, but were written before the introduction of
virBufferAdjustIndent(). They instead used the much more clunky method
of adding a "level" arg to every format function, and padding with
spaces using the "%*s" printf format specifier (giving it the level,
and "", which has the effect of adding level spaces to the output).

While eliminating the hardcoded indentation in other xml, I decided it
was finally time to also modernize the interface formatter code to
make it more consistent.