xen-netback: add software timestamp capabilities

Add software timestamp capabilities to the xen-netback driver
by advertising it on the struct ethtool_ops and calling
skb_tx_timestamp before passing the buffer to the queue.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>