xen-netback: split event channels support for Xen backend driver

Netback and netfront only use one event channel to do TX / RX notification,
which may cause unnecessary wake-up of processing routines. This patch adds a
new feature called feature-split-event-channels to netback, enabling it to
handle TX and RX events separately.

Netback will use tx_irq to notify guest for TX completion, rx_irq for RX
notification.

If frontend doesn't support this feature, tx_irq equals to rx_irq.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>