net : enable tx time stamping in the vde driver.

This new version moves the skb_tx_timestamp in the main uml
driver. This should avoid the need to call this function in each
transport (vde, slirp, tuntap, ...). It also add support for ethtool
get_ts_info.

Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>