- netdev_features_t is only available in 3.3 and later kernels. Fix
vr_compat.h to account for this.

- num_possible_cpus() can return more CPUs than are present. Use
num_present_cpus() instead.
Merge pull request #61 from srajag/master

- netdev_features_t is only available in 3.3 and later kernels. Fix
xen: Fix for GRO path

Existing vrouter code implements GRO with the option of using hard header length of the
device for GRO flow merge. This option does not exist in older kernel, where flow comparison
is based on mac header. Fixed it by adjusting the the mac header of the skb for xen kernel.

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>