device-crash-test: Ignore errors about a bus not being available

Recent QEMU versions now sometimes exit cleanly with an error message
that a bus is not available for a specified device. Don't flag those
as an error in the device-crash-test script.

Message-Id: <20210323164718.1393792-1-thuth@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>