examples/l2fwd-event: support event vector

examples/l2fwd-event: support event vector

Added changes to receive packets as event vector. By default this is
disabled and can be enabled using the option --event-vector. Vector
size and timeout to form the vector can be configured using options
--event-vector-size and --event-vector-tmo.

Example:
dpdk-l2fwd-event -l 0-3 -n 4 -- -p 0x03 --mode=eventdev \
	--eventq-sched=ordered --event-vector --event-vector-size 16

Signed-off-by: Shijith Thotton <sthotton@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>