qemu.tests: Add a test for ioeventfd of virtio block device

Do following tests with ioenventfd option for virtio block devie:
  - big file write test
  - reboot
  - iozone
  - poweroff

Yes. And it's named autotest_control in parameter 'subtests' as the script name is different from test case name. And you can find this from test_control_file below.