efi_loader: test/py: Reset system after capsule update on disk

Add a cold reset soon after processing capsule update on disk.
This is required in UEFI specification 2.9 Section 8.5.5
"Delivery of Capsules via file on Mass Storage device" as;

    In all cases that a capsule is identified for processing the system is
    restarted after capsule processing is completed.

This also reports the result of each capsule update so that the user can
notice that the capsule update has been succeeded or not from console log.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>