Comment 4 for bug 1706296

There are three possibilities:

1) push qemu_mutex_lock_iothread down to cc->do_interrupt

2) change the condition in io_readx/io_writex to mr->global_locking && !qemu_mutex_iothread_locked()

3) both

We can do (2) for 2.10 and later ponder on doing the first.