HACKING: document preference for g_new instead of g_malloc

HACKING: document preference for g_new instead of g_malloc

This patch documents the preference for g_new instead of g_malloc. The
reasons were adapted from commit b45c03f585ea9bb1af76c73e82195418c294919d.

Discussion in QEMU's mailing list:
  http://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg03238.html

Cc: [email protected]
Cc: David Hildenbrand <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: Markus Armbruster <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Murilo Opsfelder Araujo <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>