hw/riscv/riscv-iommu-sys.c: record fault on IOMMU-generated MSI write

The riscv-iommu spec requires that the IOMMU records its own generated
MSI write faults.

Fixes: 01c1caa9d1 ("hw/riscv/virt.c, riscv-iommu-sys.c: add MSIx support")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3572
Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Message-ID: <20260629165954.1018123-1-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>