[Qemu-block] Enable debugging in a running vServer

[Peter Lieven]

Hi Block people,

we recently observed some strange I/O stalls on some vServers. I suspect a bug 
in the target and
already added some debugging output to libiscsi that could have helped to track 
the issue.

However, to enable this debugging I would need to call

iscsi_set_log_level

during runtime from the hmp or qmp.

I wonder what would be the best way to do this and/or if it would be 
interesting to have a generic
way to tell a block backend to enter debugging whereas I would leave it up to 
the backend driver
what exactly that means?

Other option would be to set an enviroment variable during runtime. But as far 
as I know thats
not possible.

Any thoughts, thank you,
Peter