Fix deadlock with tap/tun devices for arm guests

[AndrewFasano]

[zestrada]

Can confirm this works for the target where we first saw the bug. There appears to be an issue with the barebox minimal test case (with or without tap device):

/out/panda-taphang/build/arm-softmmu/panda-system-arm -M vexpress-a9 -nographic -no-reboot -monitor telnet::4443,server,nowait -kernel ./images/barebox-vexpress-ca9.img -m 1024m             
audio: Could not init `oss' audio driver                                         

barebox 2022.10.0 #1 Wed Oct 19 23:03:50 UTC 2022


Board: V2P-CA9
panda-system-arm: /out/panda-taphang/memory.c:918: memory_region_transaction_commit: Assertion `qemu_mutex_iothread_locked()' failed.
Aborted (core dumped)

[zestrada]

Just a quick comment in case others run into this issue. The systems that were deadlocking were Linux kernels that somehow entered a race condition in QEMU during guest execution of the calibrate_delay kernel function. A hack to get around this has been to specify a loops per jiffy from a successful boot (e.g., without a tap device) on the kernel command line (e.g., lpj=43648). Your mileage may vary.