vhost: add vhost-user client mode

vhost: add vhost-user client mode

Add a new paramter (flags) to rte_vhost_driver_register(). DPDK
vhost-user acts as client mode when RTE_VHOST_USER_CLIENT flag
is set.  The flags would also allow future extensions without
breaking the API (again).

The rest is straingfoward then: allocate a unix socket, and
bind/listen for server, connect for client.

This extension is for vhost-user only, therefore we simply quit
and report error when any flags are given for vhost-cuse.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>