target/riscv: handle mvendorid/marchid/mimpid for KVM CPUs

target/riscv: handle mvendorid/marchid/mimpid for KVM CPUs

After changing user validation for mvendorid/marchid/mimpid to guarantee
that the value is validated on user input time, coupled with the work in
fetching KVM default values for them by using a scratch CPU, we're
certain that the values in cpu->cfg.(mvendorid|marchid|mimpid) are
already good to be written back to KVM.

There's no need to write the values back for 'host' type CPUs since the
values can't be changed, so let's do that just for generic CPUs.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>