target/riscv/kvm: change timer regs size to u64

target/riscv/kvm: change timer regs size to u64

KVM_REG_RISCV_TIMER regs are always u64 according to the KVM API, but at
this moment we'll return u32 regs if we're running a RISCV32 target.

Use the kvm_riscv_reg_id_u64() helper in RISCV_TIMER_REG() to fix it.

Reported-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20231208183835.2411523-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>