ethdev: fix behavior of VF/PF in flow API

ethdev: fix behavior of VF/PF in flow API

Contrary to all other pattern items, these are inconsistently documented as
affecting traffic instead of simply matching its origin, without provision
for the latter.

This commit clarifies documentation and updates PMDs since the original
behavior now has to be explicitly requested using the new transfer
attribute.

It breaks ABI compatibility for the following public functions:

- rte_flow_create()
- rte_flow_validate()

Impacted PMDs are bnxt and i40e, for which the VF pattern item is now only
supported when a transfer attribute is also present.

Fixes: b1a4b4cbc0a8 ("ethdev: introduce generic flow API")

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>