coverity: Model g_memdup()

We model all the non-deprecated memory allocation functions from
https://developer.gnome.org/glib/stable/glib-Memory-Allocation.html
except for g_memdup(), g_clear_pointer(), g_steal_pointer().  We don't
use the latter two.  Model the former.

Coverity now reports an OVERRUN
vl.c:2317: alloc_strlen: Allocating insufficient memory for the terminating null of the string.
Correct, but we omit the terminating null intentionally there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1448901152-11716-1-git-send-email-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
