app/eventdev: add Tx first option to pipeline mode

app/eventdev: add Tx first option to pipeline mode

Add Tx first support to pipeline mode tests, the transmission is done
on all the ethernet ports. This helps in testing eventdev performance
with standalone loopback interfaces.

Example:
./dpdk-test-eventdev ... -- ... --tx_first 512

512 defines the number of packets to transmit.
Add an option Tx packet size, the default packet size is 64.

Following example can change packet size value as 320.

Example:
./dpdk-test-eventdev ... -- ... --tx_first 512 --tx_pkt_sz 320

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>