target/riscv/kvm: read/write KVM regs via env size

target/riscv/kvm: read/write KVM regs via env size

We're going to add support for scounteren in the next patch. KVM defines
as a target_ulong CSR, while QEMU defines env->scounteren as a 32 bit
field. This will cause the current code to read/write a 64 bit CSR in a
32 bit field when running in a 64 bit CPU.

To prevent that, change the current logic to honor the size of the QEMU
storage instead of the KVM CSR reg.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20250429124421.223883-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>