qdist: use g_renew and g_new instead of g_realloc and g_malloc.

This is safer against overflow.  g_renew is available in all
version of glib, while g_realloc_n is only available in 2.24.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1469459025-23606-3-git-send-email-cota@braap.org>
[Rewritten to use g_new/g_renew. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
