fix ethdev port id validation

fix ethdev port id validation

Some DPDK applications wrongly assume these requirements:
    - no hotplug, i.e. ports are never detached
    - all allocated ports are available to the application

Such application assume a valid port index is in the range [0..count[.

There are three consequences when using such wrong design:
    - new ports having an index higher than the port count won't be valid
    - old ports being detached (RTE_ETH_DEV_UNUSED) can be valid

Such mistake will be less common with growing hotplug awareness.
All applications and examples inside this repository - except testpmd -
must be fixed to use the function rte_eth_dev_is_valid_port.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>