RISC-V: KVM: Fix KVM_GET_REG_LIST API for ISA_EXT registers

The ISA_EXT registers to enabled/disable ISA extensions for VCPU
are always available when underlying host has the corresponding
ISA extension. The copy_isa_ext_reg_indices() called by the
KVM_GET_REG_LIST API does not align with this expectation so
let's fix it.

Fixes: 031f9efa ("KVM: riscv: Add KVM_GET_REG_LIST API support")
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>