HACKING: clarify allocation/free recommendations

Clarify the allocation/free recommendations; this is mostly
just tidying up following the global-search-and-replace done
with the conversion to the GLib g_malloc and friends.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>