slirp: VMState conversion; tcpcb

Convert the migration of the struct tcpcb to use a VMStateDescription,
the rest of it will come later.

Mostly mechanical, except for conversion of some 'char' to uint8_t
to ensure portability.

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