We already have virAsprintf, so picking a similar name helps for
seeing a similar purpose.  Furthermore, the prefix V before printf
generally implies 'va_list', even though this variant was '...', and
the old name got in the way of adding a new va_list version.

global rename performed with:

$ git grep -l virBufferVSprintf \
  | xargs -L1 sed -i 's/virBufferVSprintf/virBufferAsprintf/g'

then revert the changes in ChangeLog-old.

* docs/api_extension/{0013,0014}*.patch: Rename to shorter files.
* docs/api_extension.html.in: Reflect rename.

* docs/api_extension/*: Replace example files.
* docs/api_extension.html.in: Rewrite to match new example files.