Windows.cfg: Add background command return code to ignore on cleanup

This is part of tp-qemu#149. The return code for process
not found for Windows is 128 instead of 1 (Linux). So
let's set this parameter in the Windows snippet config
file.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1800 from autotest/migration-kill-ignore-status

Windows.cfg: Add background command return code to ignore on cleanup