RISC-V: KVM: Fix riscv_vcpu_get_isa_ext_single() for missing extensions

The riscv_vcpu_get_isa_ext_single() should fail with -ENOENT error
when corresponding ISA extension is not available on the host.

Fixes: e98b1085

 ("RISC-V: KVM: Factor-out ONE_REG related code to its own source file")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>