make user networking hostfwd work with restrict=y

This patch allows the hostfwd option to override the restrict=y setting in
the user network stack, as explicitly stated in the documentation on the
restrict option:

     restrict=on|off
         If this option is enabled, the guest will be isolated, i.e. it
         will not be able to contact the host and no guest IP packets
         will be routed over the host to the outside. This option does
         not affect any explicitly set forwarding rules.

Qemu bug tracker:
https://bugs.launchpad.net/qemu/+bug/829455

Signed-off-by: Gertjan Halkes <qemu@ghalkes.nl>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
