M16: IOMMU detection (DMAR parsing)

Detect the IOMMU: discovery now parses the ACPI DMAR table, finds the first VT-d
DMA-remapping unit (DRHD), maps its register block, and records its version and
capabilities (iommu_present/base/version/capabilities in the platform info). On QEMU's
emulated intel-iommu this reads back a real unit (base 0xfed90000, version 1.0).

This is detection only, and deliberately so. A full VT-d bring-up — per-device
translation domains that confine a driver's DMA to the buffers it dma_alloc'd — is the
real device-side safety guarantee, but it cannot be verified without a DMA-capable
device driver (none exist yet) and QEMU's intel-iommu to fault against. Writing that
enforcement now would be a large body of unverifiable page-table code; it belongs with
the first DMA driver, which is both the natural order and the only way to test it. Until
then the caveat stands in full: device_claim on a DMA-capable device is still equivalent
to granting ring 0. The docs say so plainly.

New `iommu` test (harness boots it with -device intel-iommu via a new per-case qemu_extra
hook) confirms the DMAR is parsed and the unit's registers read. Suite 40/40 plus host
tests.

test/qemu_test.py

@@ -140,6 +140,12 @@ CASES = [
     {"name": "msi",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # IOMMU (M16): boot with an emulated VT-d unit and confirm danos parses the DMAR
+    # table and reads the unit's registers. Detection only — enforcement is future.
+    {"name": "iommu",
+     "qemu_extra": ["-device", "intel-iommu,intremap=off"],
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Parallelism: needs more than one core, so this case boots with -smp 4.
     {"name": "smp",
      "smp":    4,
@@ -310,6 +316,8 @@ def run_case(arch, case):
     cmd = [arch["qemu"]] + arch["qemu_args"](arch, esp, vars_fd, serial)
     if case.get("smp"):  # some cases need more than one core (e.g. parallelism)
         cmd += ["-smp", str(case["smp"])]
+    if case.get("qemu_extra"):  # extra qemu args, e.g. -device intel-iommu for the IOMMU case
+        cmd += case["qemu_extra"]
     qemu = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
     try:
         timeout = case.get("timeout", TIMEOUT)