virtio-net: introduce and use helper function for guest gso support checks

Probe routine is already several hundred lines.
Use helper function for guest gso support check.

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <20220914144911.56422-2-gavinl@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>