xen-netfront: split event channels support for Xen frontend driver

This patch adds a new feature called feature-split-event-channels for
netfront, enabling it to handle TX and RX events separately.

If netback does not support this feature, it falls back to use single event
channel.

If netfront fails to setup split event channels, it will try falling back to
single event channel.

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