tests/qemu-iotests/testenv: Use the "virt" machine for or1k

tests/qemu-iotests/testenv: Use the "virt" machine for or1k

When compiling QEMU just with "--target-list=or1k-softmmu", there
are 8 iotests failing that try to use PCI devices - but the default
or1k machine does not have a PCI bus. The "virt" machine is better
suited for running the iotests than the or1k default machine since
it provides PCI and thus e.g. support for virtio-blk and virtio-scsi,
too. With this change, there are no failing iotests anymore when
using the qemu-system-or1k binary for running the tests.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>