hw/riscv/riscv_hart.c isolate tcg only bits

riscv_cpu_register_csr_qtest_callback(), vcsr_call() and
csr_qtest_callback() are all TCG only and are not available in
--disable-tcg builds.

Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260703180538.3346781-19-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>