slirp: VMStatify socket level

Working up the stack, this replaces the slirp_socket_load/save
with VMState definitions.

A place holder for IPv6 support is added as a comment; it needs
testing once the rest of the IPv6 code is there.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>