KVM: riscv: selftests: Align the trap information wiht pt_regs

KVM: riscv: selftests: Align the trap information wiht pt_regs

The current exeception register structure in selftests are missing
few registers (e.g stval). Instead of adding it manually, change
the ex_regs to align with pt_regs to make it future proof.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250430-kvm_selftest_improve-v3-1-eea270ff080b@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>