qemu/tests/migration.py: Ignore background command kill failures at cleanup

If the cleanup command returns 1, it means the migrate
background process is already gone, and a failure
already happened in that particular migration test.

So, just ignore this specific failure at the cleanup
stage.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu/tests/migration.py: Log extra info during migration

Add the appropriate context when we're checking the status
of the background processes pre and post migration.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>