slirp: Allow non-local DNS address when restrict is off

This can be used to set a DNS server to be used by the guest which is
different from the one configured on the host.

Buglink: https://bugs.launchpad.net/qemu/+bug/1010484


Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>