When migration fails, QEMU exits with a status code EXIT_FAILURE.
Change qtests to use the well-defined macro instead of magic number.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20221028045736.679903-6-bin.meng@windriver.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>