examples/multi_process: add hotplug sample

examples/multi_process: add hotplug sample

The sample code demonstrates device (ethdev only) management
at a multi-process environment. The user can attach/detach a
device on primary process and see it is synced on secondary
process automatically.

How to start?
./hotplug_mp --proc-type=auto

Command Line Example:

>help
>list

/* attach a pci device */
> attach 0000:81:00.0

/* detach the pci device */
> detach 0000:81:00.0

/* attach a vdev af_packet device */
> attach net_af_packet,iface=eth0

/* detach the vdev af_packet device */
> detach net_af_packet

Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>