LoadVM assert on ARM_FEATURE_M for Cortex M3
Description of problem:
This appears to be a similar issue to https://gitlab.com/qemu-project/qemu/-/issues/1775 and https://gitlab.com/qemu-project/qemu/-/issues/1658

When running `loadvm`  qemu aborts with this error:

"qemu/target/arm/helper.c:12383: arm_security_space_below_el3: Assertion `!arm_feature(env, ARM_FEATURE_M)' failed."

I've traced the error to `pmu_counter_enabled` in `qemu\target\arm\helper.c:1172`   
 [uint64_t mdcr_el2 = arm_mdcr_el2_eff(env)](https://gitlab.com/qemu-project/qemu/-/blob/v8.2.0/target/arm/helper.c?ref_type=tags#L1172)  (link is to 8.2.0 release tag)


The issue is caused by attempting to get the MDCR_EL2 register  prior to checking if the CPU has ARM_FEATURE_PMU support. 

A simple fix seems to be to check for `ARM_PMU_ENABLED` and returning early if it is not enabled.
Steps to reproduce:
1. Start emulation and connect monitor
2. savevm <snapshot-name>
3. Loadvm <snapshot-name>
Additional information:
See screenshot for stack trace

![armCortexM3LoadVMStackTrace](/uploads/fcfd927f4d373922715c8787dbb9cc26/armCortexM3LoadVMStackTrace.png)