ethdev: allow returning error on VLAN offload ops

Some devices may not support or fail setting VLAN offload
configuration based on dynamic circumstances so the
vlan_offload_set_t vector is modified to return an int so
the caller can determine success or not.

rte_eth_dev_set_vlan_offload is updated to return the
value provided by the vector when called along with restoring
the original offload configs on failure.

Existing vlan_offload_set_t vectors are modified to return
an int.  Majority of cases return 0 but a few that actually
can fail now return their failure codes.

Finally, a vlan_offload_set_t vector is added to virtio
to facilitate dynamically turning VLAN strip on or off.

Signed-off-by: David Harton <dharton@cisco.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>