hw/char: riscv_htif: Move registers from CPUArchState to HTIFState

hw/char: riscv_htif: Move registers from CPUArchState to HTIFState

At present for some unknown reason the HTIF registers (fromhost &
tohost) are defined in the RISC-V CPUArchState. It should really
be put in the HTIFState struct as it is only meaningful to HTIF.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221229091828.1945072-6-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>