vdpa/mlx5: Extend driver support for new features

Extend the possible list for features that can be supported by firmware.
Note that different versions of firmware may or may not support these
features. The driver is made aware of them by querying the firmware.

While doing this, improve the code so we use enum names instead of hard
coded numerical values.

The new features supported by the driver are the following:

VIRTIO_NET_F_MRG_RXBUF
VIRTIO_NET_F_HOST_ECN
VIRTIO_NET_F_GUEST_ECN
VIRTIO_NET_F_GUEST_TSO6
VIRTIO_NET_F_GUEST_TSO4

Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20230321112809.221432-3-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez Martin <eperezma@redhat.com>