maint: rename virBufferVSprintf to virBufferAsprintf

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.

HACKING

@@ -545,7 +545,7 @@ Typical usage is as follows:
      ...
 
      virBufferAddLit(&buf, "<domain>\n");
-     virBufferVSprintf(&buf, "  <memory>%d</memory>\n", memory);
+     virBufferAsprintf(&buf, "  <memory>%d</memory>\n", memory);
      ...
      virBufferAddLit(&buf, "</domain>\n");