[Qemu-block] Clean Block Driver Shutdown

[Peter Lieven]

Hi,

I noticed that Qemu quits at several points with an exit() if the supplied 
parameters in the commandline are incorrect.
This at some stages happens after there have already been connections to 
storage backends established.
These connections are not cleanly shut down in this case. For posix file 
backends that doesn't matter, but for other
backends this leads to errors. E.g. iSCSI Targets log an aborted iSCSI 
connection due to tcp reset.

I wonder what is the best way to fix this. A simply call to bdrv_close_all() in 
an atexit handler seems to work.
But is this a good solution? Maybe register this handler only until the VM 
starts.
Or do we need an atexit handler in each block driver that requires a clean 
shutdown?

Thanks,
Peter