target/riscv/tcg-cpu.c: add extension properties for all cpus

target/riscv/tcg-cpu.c: add extension properties for all cpus

At this moment we do not expose extension properties for vendor CPUs
because that would allow users to change them via command line. The
drawback is that if we were to add an API that shows all CPU properties,
e.g. qmp-query-cpu-model-expansion, we won't be able to show extensions
state of vendor CPUs.

We have the required machinery to create extension properties for vendor
CPUs while not allowing users to enable extensions. Disabling existing
extensions is allowed since it can be useful for debugging.

Change the set() callback cpu_set_multi_ext_cfg() to allow enabling
extensions only for generic CPUs. In cpu_add_multi_ext_prop() let's not
set the default values for the properties if we're not dealing with
generic CPUs, otherwise the values set in cpu_init() of vendor CPUs will
be overwritten. And finally, in tcg_cpu_instance_init(), add cpu user
properties for all CPUs.

For the veyron-v1 CPU, we're now able to disable existing extensions
like smstateen:

$ ./build/qemu-system-riscv64 --nographic -M virt \
    -cpu veyron-v1,smstateen=false

But setting extensions that the CPU didn't set during cpu_init(), like
V, is not allowed:

$ ./build/qemu-system-riscv64 --nographic -M virt \
    -cpu veyron-v1,v=true
qemu-system-riscv64: can't apply global veyron-v1-riscv-cpu.v=true:
  'veyron-v1' CPU does not allow enabling extensions

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230926183109.165878-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>