virtio-gpu: a modern virtio 1.0 display driver, bring-up to a flushed frame (v2 V3)

The first native scanout backend's driver half. The device manager matches the
virtio-gpu PCI function by its Display/Other class triple and spawns the driver,
which claims the device, confirms vendor 0x1AF4/device 0x1050 from config space,
enables memory-space + bus mastering, and walks the virtio vendor capabilities to
find the common-config and notify structures in its BAR.

From there it is the standard modern-virtio bring-up: reset, negotiate VERSION_1,
stand up the control virtqueue in coherent DMA, then drive the GPU end to end —
RESOURCE_CREATE_2D, ATTACH_BACKING (a coherent DMA buffer; V4 swaps in the shm-
shared surface), SET_SCANOUT, paint a known pattern, TRANSFER_TO_HOST_2D,
RESOURCE_FLUSH, and wait for the device's used-ring ack. Reading the backing back
proves it is CPU-visible RAM; the ack proves the device consumed the frame —
together the automated stand-in for "it's on screen", no screenshot.

- system/drivers/virtio-gpu/: the driver, plus virtio-gpu-protocol.zig (control
  commands) and virtio-pci.zig (the 1.0 PCI transport + split-virtqueue), both with
  host-tested struct sizes.
- device-manager matches the display/other class triple to "virtio-gpu"; the driver
  self-confirms the vendor/device id, since the class alone cannot distinguish it.
- ServiceId.scanout (11): the driver registers it so the compositor finds it in V4.

Gate: python3 test/qemu_test.py virtio-gpu (QEMU -device virtio-gpu-pci) — the
device-manager stack discovers the function, the driver brings up a 640x480 scanout
and flushes a test pattern: "virtio-gpu: scanout 640x480 online" + "flush acked,
pixel check ok". host tests, display-service, shm, and device-list still pass.

Note: the pre-existing pci-scan case triple-faults on main (verified at 88ad432,
before this change); it is unrelated and tracked separately.

test/qemu_test.py

@@ -191,6 +191,15 @@ CASES = [
     {"name": "shm",
      "expect": r"shm: shared 4096 bytes ok",
      "fail":   r"shm: (shared FAILED|create failed|no server|call failed|map)|CPU EXCEPTION|KERNEL PANIC"},
+    # virtio-gpu driver (v2 V3): boot with an emulated virtio-gpu. The device-manager stack
+    # discovers the PCI function and spawns the driver, which brings up the control virtqueue,
+    # creates a 2D scanout resource backed by DMA memory, set_scanouts it, paints a test
+    # pattern, transfers + flushes it, and waits for the device's used-ring ack, then reads
+    # the backing back. `scanout WxH online` + `flush acked, pixel check ok` are the markers.
+    {"name": "virtio-gpu",
+     "qemu_extra": ["-device", "virtio-gpu-pci"],
+     "expect": r"virtio-gpu: scanout \d+x\d+ online[\s\S]*virtio-gpu: flush acked, pixel check ok",
+     "fail":   r"virtio-gpu:.*(failed|not acked|mismatch|unable to claim|not a virtio-gpu|too small|no PCI capability|does not offer|rejected|missing common-config|not a mapped resource|could not spawn)|CPU EXCEPTION|KERNEL PANIC"},
     # Monotonic clock (clock() syscall source): calibrated, advancing, never backwards.
     {"name": "clock",
      "expect": r"DANOS-TEST-RESULT: PASS",