eventdev: introduce event vector Tx capability

eventdev: introduce event vector Tx capability

Introduce event vector transmit capability for event eth
tx adapter.

The capability indicates that the Tx adapter is capable of
transmitting event vectors.
When rte_event_vector::union_valid is set, the Tx adapter should
transmit all the packets to the rte_event_vector::port using the
rte_event_vector::queue.
If rte_event_vector::union_valid is not set then the Tx adapter
should peek into each mbuf to get the destination port and queue
pair.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>