vhost: add reconnect ability

vhost: add reconnect ability

Allow reconnecting on failure by default when:

- DPDK app starts first and QEMU (as the server) is not started yet.
  Without reconnecting, DPDK app would simply fail on vhost-user
  registration.

- QEMU restarts, say due to OS reboot.
  Without reconnecting, you can't re-establish the connection without
  restarting DPDK app.

This patch make it work well for both above cases. It simply creates
a new thread, and keep trying calling "connect()", until it succeeds.

The reconnect could be disabled when RTE_VHOST_USER_NO_RECONNECT flag
is set.

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