slirp: Make Socket structure IPv6 compatible

This patch replaces foreign and local address/port couples in Socket
structure by 2 sockaddr_storage which can be casted in sockaddr_in.
Direct access to address and port is still possible thanks to some
\#define, so retrocompatibility of the existing code is assured.

The ss_family field of sockaddr_storage is declared after each socket
creation.

The whole structure is also saved/restored when a Qemu session is
saved/restored.

This prepares for IPv6 support.

Signed-off-by: Guillaume Subiron <maethor@subiron.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>