target/arm: cannot boot when CPU supports SME
Description of problem:
On macOS 15.2 beta, Apple's Hypervisor.framework exposes the SME feat flag to QEMU. As a result, in `arm_cpu_sme_finalize`, `cpu_isar_feature(aa64_sme, cpu)` returns true and the program will always exit with the following:

```
qemu-aarch64-softmmu: cannot disable sme4224
All SME vector lengths are disabled.
With SME enabled, at least one vector length must be enabled.
```

This is because `vq_supported` and `vq_init` are both 0 as they are not initialized anywhere. It seems that in the original commit e74c097638d38b46d9c68f11565432034afc0ad0 the only place `cpu->sme_vq.supported` is initialized is with `aarch64_max_initfn` when KVM and HVF are not used as the backend.