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Monitor device_add triggers deadlock when calling drain_call_rcu on QEMU >= 6.0.0
Description of problem:
It hangs
Steps to reproduce:
1. Run the QEMU:
```
./qemu-system-mips64 -nographic
```
2. Enter into the QEMU monitor: press ctrl-a c
3. Execute command `device_add` without arguments:
```
(qemu) device_add
```
4. It hangs so bad that only `kill -9` helps
Additional information:
I didn't test versions between 4.2.0 and 6.0.0, but I can confirm that 6.0.0, 6.1.0 and the latest master pull have this bug, while version 4.2.0 doesn't have it.
I've tracked the problem and found this.
1. Command `device_add` calls function `drain_call_rcu`. `drain_call_rcu` waits indefinitely for drain_complete_event.
2. Function `cpu_exec` in accel/tcg/cpu-exec.c calls `rcu_read_lock` but does not call `rcu_read_unlock()`. `cpu_exec` just spins in its inner loop.
3. Function `call_rcu_thread` hanged in calling the `synchronize_rcu` which calls `wait_for_readers`.
If I execute `stop` command in QEMU monitor before calling `device_add` command, no hang happen.
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