target/riscv/kvm: change KVM_REG_RISCV_FP_D to u64

target/riscv/kvm: change KVM_REG_RISCV_FP_D to u64

KVM_REG_RISCV_FP_D regs are always u64 size. Using kvm_riscv_reg_id() in
RISCV_FP_D_REG() ends up encoding the wrong size if we're running with
TARGET_RISCV32.

Create a new helper that returns a KVM ID with u64 size and use it with
RISCV_FP_D_REG().

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: <20231208183835.2411523-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>