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.

system/abi.zig

@@ -187,6 +187,7 @@ pub const ServiceId = enum(u32) {
     fat = 8, // the FAT filesystem server; the VFS mounts it and forwards paths under its mount point (/mnt/usb) to it
     display = 9, // the display service: owns the framebuffer, composites a layer stack, presents frames (docs/display.md)
     shm_test = 10, // the shm test server (V2): a client passes it a shared-memory capability, it maps + verifies (docs/display-v2.md)
+    scanout = 11, // a native scanout driver (virtio-gpu): the compositor finds it here to upgrade off the GOP framebuffer (docs/display-v2.md)
     _,
 };