util: buffer: Introduce VIR_AUTOCLEAN function for virBuffer

util: buffer: Introduce VIR_AUTOCLEAN function for virBuffer

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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>