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

If the cleanup command returns 1 on Linux, 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. The problem is that under windows, the appropriate
error code is 128 [1]. So we have to make the ignore error
code configurable, and change the windows config file in
virt-test (on a separate PR).

[1] http://www.symantec.com/connect/forums/error-128-during-script-execution

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>