KVM: riscv: selftests: Fix ISA_EXT register handling in get-reg-list

Same set of ISA_EXT registers are not present on all host because
ISA_EXT registers are visible to the KVM user space based on the
ISA extensions available on the host. Also, disabling an ISA
extension using corresponding ISA_EXT register does not affect
the visibility of the ISA_EXT register itself.

Based on the above, we should filter-out all ISA_EXT registers.

Fixes: 47706939 ("KVM: riscv: selftests: Add get-reg-list test")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>