[Qemu-block] Crash when deleting an iothread that is being used

[Alberto Garcia]

Here's how to reproduce the crash:

{ "execute": "qmp_capabilities" }
{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "node-name": 
"hd0"}}
{ "execute": "object-add", "arguments": {"qom-type": "iothread", "id": 
"iothread0"}}
{ "execute": "x-blockdev-set-iothread", "arguments": {"node-name": "hd0", 
"iothread": "iothread0"}}
{ "execute": "object-del", "arguments": {"id": "iothread0"}}
{ "execute": "blockdev-del", "arguments": {"node-name": "hd0"}}

The problem is that bs->aio_context is the one that belonged to the
IOThread and was destroyed by the object-del call. One would need to
do x-blockdev-set-iothread(hd0, null) before deleting the thread.

The IOThread class does not have a can_be_deleted() method to prevent
threads from being deleted. One possible implementation would require
a reference count but that doesn't seem immediately trivial because
users don't use the IOThread itself but its AioContext, and not all
bdrv_set_aio_context() are related to IOThreads.

A quicker fix is of course to prevent the threads from being deleted
at all :-)

Berto