architecture: 0.963
graphic: 0.937
arm: 0.874
debug: 0.854
performance: 0.807
virtual: 0.769
device: 0.747
semantic: 0.712
kernel: 0.703
peripherals: 0.566
user-level: 0.564
TCG: 0.531
PID: 0.441
hypervisor: 0.417
boot: 0.407
files: 0.383
register: 0.307
ppc: 0.281
risc-v: 0.255
mistranslation: 0.196
permissions: 0.173
VMM: 0.168
network: 0.148
assembly: 0.141
vnc: 0.137
socket: 0.065
x86: 0.020
i386: 0.005
KVM: 0.003

Assert error with accel=hvf:tcg when hvf is unavailable
Description of problem:
Trying to start qemu with `-machine virt,accel=hvf:tcg` in a Mac OS guest under Mac OS host, both arm64. I expect it to try hvf (unavailable - nested virtualization not supported) and fallback to tcg. Documentation for accel option says "If there is more than one accelerator specified, the next one is used if the previous one fails to initialize." This works fine with kvm, but for hvf it crashes in some auxiliary function when trying it:

```
% qemu-system-aarch64 -machine virt,accel=hvf:tcg
qemu-system-aarch64: Error: ret = HV_UNSUPPORTED (0xfae9400f, at ../target/arm/hvf/hvf.c:949)
```

Stack trace
```
  * frame #0: 0x0000000193a18388 libsystem_kernel.dylib`__pthread_kill + 8
    frame #1: 0x0000000193a5188c libsystem_pthread.dylib`pthread_kill + 296
    frame #2: 0x000000019395ac60 libsystem_c.dylib`abort + 124
    frame #3: 0x00000001005ee7f4 qemu-system-aarch64`assert_hvf_ok_impl + 92
    frame #4: 0x000000010032a550 qemu-system-aarch64`hvf_arm_get_max_ipa_bit_size + 64
    frame #5: 0x0000000100334928 qemu-system-aarch64`virt_hvf_get_physical_address_range + 68
    frame #6: 0x00000001005ee9b8 qemu-system-aarch64`hvf_accel_init + 68
    frame #7: 0x00000001002ef8e4 qemu-system-aarch64`accel_init_machine + 92
    frame #8: 0x00000001002a6640 qemu-system-aarch64`do_configure_accelerator + 208
    frame #9: 0x0000000100782bdc qemu-system-aarch64`qemu_opts_foreach + 112
    frame #10: 0x00000001002a3180 qemu-system-aarch64`qemu_init + 11344
    frame #11: 0x00000001006ea76c qemu-system-aarch64`main + 36
    frame #12: 0x00000001936b2b4c dyld`start + 6000
```