bdrv_read/write_em will cause cpu thread call aio callback and bottom half. This
is problematic and finding places to protect for concurrent access is really
painful. So I make the emulated sync. IO really synchronous.
Add a flag in QEMUIOVector to mark the operation as emulating synchronous IO.
Then in qcow2_aio_read/write_cb, do synchronous IO by recursive call and using
sync. bdrv_read/write operation.