tests/migration: Print some debug on bad status

tests/migration: Print some debug on bad status

We're seeing occasional asserts in 'wait_for_migraiton_fail', that
I can't reliably reproduce, and where the cores don't have any useful
state.  Print the 'status' out, so we can see which unexpected state
we're ending up in.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20191108104307.125020-1-dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>