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: qemu-devel@nongnu.org
Cc: David Hildenbrand <david@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>