common: Fix multiple logical errors in get_device_id.

common: Fix multiple logical errors in get_device_id.

0. If blkdev.serial exists, the serial should be used. The original impl seems wrong (if serial does not exist, then use the value from the uninitialized buffer, or even worse, use the value from the last call (model))
1. When using fallback methods, device id should only be returned when both model and serial are present. The original impl looks like a logical error.

Signed-off-by: Difan Zhang <difan@google.com>