v6.2.0 armv7m: savevm fails assertion
Description of problem:
Trying to take a snapshot on some arm machines just fails an assertion, while some work fine.  
e.g. mps2-an385 and stm32vldiscovery don't work, while e.g. raspi0 does.
```
$ build/qemu-system-arm -machine mps2-an385 -monitor stdio -drive file=dummy.qcow2 -S 
QEMU 6.1.50 monitor - type 'help' for more information
(qemu) VNC server running on ::1:5900
savevm test
qemu-system-arm: ../migration/vmstate.c:363: vmstate_save_state_v: Assertion `first_elem || !n_elems || !size' failed.
[1]    631940 IOT instruction (core dumped)  build/qemu-system-arm -machine mps2-an385 -monitor stdio -drive  -S
```
This happens with or without a kernel (so -S is optional, if a kernel is present).
Steps to reproduce:
1. Create some image for snapshots (once): ``qemu-img create -f qcow2 dummy.qcow2 32M``
2. ``qemu-system-arm -machine mps2-an385 -monitor stdio -drive file=dummy.qcow2 -S``
3. In monitor: ``savevm something``
Additional information:
Bisect indicates the Problem first presented itself in commit d5093d961585f02126191951ded9b90dbc52883b by @pm215.  
This led me to test stm32vldiscovery, which also includes armv7m.h and fails, while some others don't.