slirp: VMStatify sbuf

Convert the sbuf structure to a VMStateDescription.
Note this uses the VMSTATE_WITH_TMP mechanism to calculate
and reload the offsets based on the pointers.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>