conf: simplify Buffer Indentation in virDomainNetDefFormat

conf: simplify Buffer Indentation in virDomainNetDefFormat

This function has several calls to increase the buffer indent by 6,
then decrease it again, then increase, then decrease. Additionally,
there were several printfs that had 6 spaces at the beginning of the
line.

virDomainActualNetDefFormat, which is called by virDomainNetDefFormat,
had similar ugliness.

This patch changes both functions to just increase the indent at the
beginning, decrease it at (well, just before*) the end, and remove all
of the occurences of 6/8 spaces at the beginning of lines.

*The indent had to be reset before the end of the function because
virDomainDeviceInfoFormat assumes a 0 indent and is called from many
other places, and I didn't want to do an overhaul of every caller of
that function. A separate patch to switch all of domain_conf.c would
be a useful exercise, but my current goal is unrelated to that, so
I'll leave it for another day.