net/i40e: support floating VEB

The standard Virtual Ethernet Bridge(VEB) definition in 1Qbg is a bridge
which has an uplink port to the outside world (maybe another bridge),
but a "floating" VEB is a special VEB without an uplink port to the
outside. Instead, traffic can be sent from one VF to another using the
floating VEB - even when the physical link on the NIC port is down.

VFs VSIs connect either to the standard VEB/VEPA or to the floating VEB,
they cannot connect to both of them. The PF, VMDQ and FD VSIs still
connect to the normal VEB/VEPA.

Signed-off-by: Zhe Tao <zhe.tao@intel.com>