New Xen netback implementation

A new netback implementation which includes three major features:

 - Global page pool support
 - NAPI + kthread 1:1 model
 - Netback internal name changes

This patch series is the foundation of furture work. So it is better
to get it right first. Patch 1 and 3 have the real meat.

The first benifit of 1:1 model will be scheduling fairness.

The rational behind a global page pool is that we need to limit
overall RAM consumed by all vifs.

Utilization of NAPI enables the possibility to mitigate
interrupts/events, but this is not yet done.

Netback internal changes cleans up the code structure after switching
to 1:1 model. It also prepares netback for further code layout
changes.


---
 drivers/net/xen-netback/Makefile    |    2 +-
 drivers/net/xen-netback/common.h    |   94 +++--
 drivers/net/xen-netback/interface.c |  115 ++++--
 drivers/net/xen-netback/netback.c   |  743 +++++++++++------------------------
 drivers/net/xen-netback/page_pool.c |  183 +++++++++
 drivers/net/xen-netback/page_pool.h |   61 +++
 drivers/net/xen-netback/xenbus.c    |    6 +-
 7 files changed, 620 insertions(+), 584 deletions(-)