hyperv: Add netpoll support

In order to have at least a netconsole to debug kernel issues on
Windows Azure this patch implements netpoll support.
Sending packets is easy, netvsc_start_xmit() does already everything
needed.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>