scripts/qemu.py: log QEMU launch command line

Even when the launch of QEMU succeeds, it's useful to have the command
line recorded.

Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20190202005610.24048-2-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>