PV-on-HVM: Conditionally disable checksum offload for receive on some

PV-on-HVM: Conditionally disable checksum offload for receive on some
older kernels.

On some older kernels the CHECKSUM_UNNECESSARY flag on received
skbuffs is not honoured by all subsystems, specifically netfilter
under 2.6.9-42.EL

To work around this we need to ensure that the correct checksum is
calculated at some point. We can cause the backend to do this for us
by advertising to the backend that we cannot support checksum offload.

The correlation of this issue with the presence of NETIF_F_TSO is not
100% but it is a good first approximation.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>