pick fix for potential deadlock with QMP resize and iothread

pick fix for potential deadlock with QMP resize and iothread

While the patch gives bdrv_graph_wrlock() as an example where the
issue can manifest, something similar can happen even when that is
disabled. Was able to reproduce the issue with
while true; do qm resize 115 scsi0 +4M; sleep 1; done
while running
fio --name=make-mirror-work --size=100M --direct=1 --rw=randwrite \
 --bs=4k --ioengine=psync --numjobs=5 --runtime=1200 --time_based
in the VM.

Fix picked up from:
https://lists.nongnu.org/archive/html/qemu-devel/2023-12/msg01102.html

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>