This allows flexibility to be reused for all kinds of command line
used by other tests.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-id: 20181204035347.6148-4-jasowang@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>