target/riscv/tcg: sret in virtual user mode raises virtual instruction exception

Currently, when a `sret` is executed in virtual user mode, qemu raise an `illegal instruction exception`, but in this case the correct behavior is to raise a `virtual instruction exception` and the code already contains a check to it, but it's not enough to catch. This patch is useful to improve the correctness of the virtualization of the risc v architecture. Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3622 Signed-off-by: Christian S. Lima <christianslima@proton.me> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Message-ID: <20260808031849.59726-1-christianslima@proton.me> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>