virbuf: add auto-indentation support

virbuf: add auto-indentation support

Rather than having to adjust all callers in a chain to deal with
indentation, it is nicer to have virBuffer do auto-indentation.

* src/util/buf.h (_virBuffer): Increase size.
(virBufferAdjustIndent, virBufferGetIndent): New prototypes.
* src/libvirt_private.syms (buf.h): Export new functions.
* src/util/buf.c (virBufferAdjustIndent, virBufferGetIndent): New
functions.
(virBufferSetError, virBufferAdd, virBufferAddChar)
(virBufferVasprintf, virBufferStrcat, virBufferURIEncodeString):
Implement auto-indentation.
* tests/virbuftest.c (testBufAutoIndent): Test it.
(testBufInfiniteLoop): Don't rely on internals.
Idea by Daniel P. Berrange.