Merge branch 'xen_split_event_channels'

Wei Liu says:

====================
This series adds a new feature called split event channels.

In the original implementation, only one event channel is setup between
frontend and backend. This is not ideal as TX notification interferes with RX
notification. Using dedicated event channels for TX and RX solves this issue.

Changes since V2:
* feature_split_event_channels -> separate_tx_rx_irq
* make separate_tx_rx_irq bool
* document this feature in header file
* don't report fatal if writing feature to xenstore fails
* frontend will fall back to single event channel if new feature setup fails

Changes since V1:
* change subject lines of commits to be more specific
* add parameter feature_split_event_channels for xen-netback
* remove two dev_info
====================

Signed-off-by: David S. Miller <davem@davemloft.net>