target/riscv: Restrict KVM-specific fields from ArchCPU

target/riscv: Restrict KVM-specific fields from ArchCPU

These fields shouldn't be accessed when KVM is not available.

Restrict the KVM timer migration state. Rename the KVM timer
post_load() handler accordingly, because cpu_post_load() is
too generic.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230626232007.8933-3-philmd@linaro.org>