qemu-iotests: add 039 qcow2 lazy refcounts test

qemu-iotests: add 039 qcow2 lazy refcounts test

This tests establishes the basic post-conditions of the qcow2 lazy
refcounts features:

  1. If the image was closed normally, it is marked clean.

  2. If an allocating write was performed and the image was not closed
     normally, then it is marked dirty.

     a. Written data can be read back successfully.
     b. The image file can be repaired and will be marked clean again.
     c. The image file is automatically repaired when opened read/write.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>