virBuffer is almost always stack-allocated, but requires freeing of the
internals on error. Introduce a VIR_AUTOCLEAN function to deal with
this.

Along with the addition add a test which would leak the buffer contents
if it weren't autocleaned.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>