common/iavf: support VLAN offload by DCF

common/iavf: support VLAN offload by DCF

Add new opcode VIRTCHNL_OP_DCF_VLAN_OFFLOAD to set VLAN offload
by DCF, the virtchnl message includes:
1. A valid target VF
2. Type of VLAN to be supported: outer or inner
3. Ethertype of the VLAN (either 0x8100 or 0x88A8 or 0x9100)
4. VLAN insert settings
   a). No insert offload, VLAN ID in the packet (default)
   b). Offload via transmit descriptor
   c). Insert as a port VLAN (via VSI)
5. VLAN strip settings
   a). Strip (and discard)
   b). Strip and place in descriptor
   c). No Strip
6. VLAN ID for the target VF

Signed-off-by: Qiming Yang <qiming.yang@intel.com>
Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>