vmbus: unregister device_obj->channels_kset

Without the patch, a device can't be thoroughly destroyed, because
vmbus_device_register() -> kset_create_and_add() still holds a reference
to the hv_device's device.kobj.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Fixes: c2e5df61 ("vmbus: add per-channel sysfs info")
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>