tests/qemu-iotests/check: Pick a default machine if necessary

tests/qemu-iotests/check: Pick a default machine if necessary

qemu-system-arm, qemu-system-aarch64 and qemu-system-tricore do not have
a default machine, so when running the qemu-iotests with such a binary,
lots of tests are failing. Fix it by picking a default machine in the
"check" script instead.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20190502084506.8009-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>