rte_vhost: fix compilation against dpdk master

struct ether_addr was renamed to struct rte_ether_addr
in latest DPDK master, but our internal fork of rte_vhost
still used the old name, which can be now a non-defined type.

Together with the struct, the RTE_ETHER_ADDR_LEN define
was renamed as well, so we'll now check if it's defined and
we'll manually define struct ether_addr to keep the old
rte_vhost working.

Change-Id: I78b8104ed3bfe03397881a94f0f8bee14f9efae8
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/457609


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>