slirp: improve send_packet() callback

Use a more descriptive name for the callback.

Reuse the SlirpWriteCb type. Wrap it to check that all data has been written.

Return a ssize_t for potential error handling and data-loss reporting.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>