xen/netfront: add netconsole support.

add polling interface to xen-netfront device to support netconsole

This patch also alters the spin_lock usage to use irqsave variant.
Documentation/networking/netdevices.txt states that start_xmit
can be called with interrupts disabled by netconsole and therefore using
the irqsave/restore locking in this function is looks correct.

Signed-off-by: Tina.Yang <tina.yang@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Zhenzhong.Duan <zhenzhong.duan@oracle.com>
Tested-by: gurudas.pai <gurudas.pai@oracle.com>
[v1: Copy-n-pasted Ian Campbell comments]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>