conf: eliminate hardcoded indentation in nwfilter xml

conf: eliminate hardcoded indentation in nwfilter xml

This file was using multiple virBuffers, inserting the contents of
buf3 into buf2, then inserting the contents of buf2 into buf1, rather
than the more conventional method of just passing around a single
virBufferPtr and streaming everything into that single buffer. This
was unnecessary, and also made it more difficult to make indentation
relative, because when you insert a string into a buffer, the
indentation of the buffer is only applied once at the beginning of the
string, *not* each time a newline is encountered in the string.