Please try disabling I/O limits on the drive and try again.
Is there anything else I could try? I've captured trace of hanged VM with following events traced: bdrv_* paio_* thread_pool_* commit_* qcow2_*and debug code that prints requests from traced_requests in bdrv_requests_pending function.
It's available here: http://filebin.net/tmscfay2pa/hanged-trace.gz (50mb after decompression)
-- mg
