slirp: Fix migration from older versions of QEMU to the current one

While adding the IPv6 support, the commit eae303ff23f51259eddc8856c71453d8
("slirp: Make Socket structure IPv6 compatible") changed the format of
the migration stream, without taking into account that we might still
receive an old migration stream layout when upgrading from QEMU version
2.5 (or older) to QEMU 2.6. Currently, QEMU bails out when doing a
migration from QEMU 2.5 to the recent master version when it has
been started with a "-net user,guestfwd=..." network. So let's fix
this by checking the version ID of the migration stream and by using
the old behavior if we've detected version 3 or less.

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