bus/vdev: scan by multi-process channel

bus/vdev: scan by multi-process channel

To scan the vdevs in primary, we send request to primary process
to obtain the names for vdevs.

Only the name is shared from the primary. In probe(), the device
driver is supposed to locate (or request more) the detail
information from the primary.

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Qi Zhang <qi.z.zhang@intel.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>