target/riscv/cpu.c: do not run 'host' CPU with TCG

The 'host' CPU is available in a CONFIG_KVM build and it's currently
available for all accels, but is a KVM only CPU. This means that in a
RISC-V KVM capable host we can do things like this:

$ ./build/qemu-system-riscv64 -M virt,accel=tcg -cpu host --nographic
qemu-system-riscv64: H extension requires priv spec 1.12.0

This CPU does not have a priv spec because we don't filter its extensions
via priv spec. We shouldn't be reaching riscv_cpu_realize_tcg() at all
with the 'host' CPU.

We don't have a way to filter the 'host' CPU out of the available CPU
options (-cpu help) if the build includes both KVM and TCG. What we can
do is to error out during riscv_cpu_realize_tcg() if the user chooses
the 'host' CPU with accel=tcg:

$ ./build/qemu-system-riscv64 -M virt,accel=tcg -cpu host --nographic
qemu-system-riscv64: 'host' CPU is not compatible with TCG acceleration

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230721133411.474105-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>