ethdev: add port representor device flag

Add new device flag to specify that an ethdev port is a port
representor.  Extend rte_eth_dev_info structure to expose device flags
to the user which enables applications to discover if a port is a
representor port.

Signed-off-by: Declan Doherty <declan.doherty@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>