iotests: use iotests.VM in 238

iotests: use iotests.VM in 238

Test 238 does not require the kvm accelerator.  Using the qtest
accelerator allows the test to run in both non-kvm and non-tcg
environments.

iotests.VM implicitly uses the qtest accelerator and is really the class
that this test should be using.  Switch to that instead of
qemu.QEMUMachine.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>