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

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #94 from ypu/ioeventfd

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