virbuffer: Set child buffer indent properly

virbuffer: Set child buffer indent properly

There's this macro virBufferSetChildIndent which sets offset of
child buffer from given parent buffer. However, it is calling
virBufferAdjustIndent() which only adds adjustment instead of
calling virBufferSetIndent() which clears out any adjustment
previously set.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>