refactor: extract shared hw-virtio crate, eliminate VirtIO protocol duplication

Two VirtIO block and two VirtIO GPU implementations (adapter + driver)
shared 90% of their protocol logic. Extracted the common code into
hw-virtio, parameterized by MmioAccess trait.

New crate: hw-virtio (no_std, zero deps)
  - mmio.rs: MmioAccess trait, DirectMmio, init_legacy_device(), scan
  - queue.rs: VirtioQueue with descriptor chain submit + poll
  - block.rs: block read protocol (3-descriptor chain)
  - gpu.rs: GPU commands (create_2d, attach_backing, set_scanout,
            transfer_to_host, resource_flush)

Adapter thin wrappers (identity-mapped, statics for DMA):
  - virtio_blk.rs: 220 → 53 lines
  - virtio_gpu.rs: 165 → 37 lines

Driver thin wrappers (mapped VAs, phys-info for DMA):
  - drivers/block/virtio: 139 → 70 lines
  - drivers/framebuffer/virtio: 146 → 95 lines

Also deleted input-server/input_device.rs (1-line re-export,
superseded by port-input-device).

Cargo.lock

@@ -12,6 +12,7 @@ version = "0.1.0"
 dependencies = [
  "domain",
  "elf",
+ "hw-virtio",
  "kernel",
  "platform",
  "types",
@@ -81,6 +82,7 @@ dependencies = [
 name = "driver-block-virtio"
 version = "0.1.0"
 dependencies = [
+ "hw-virtio",
  "port-block-device",
 ]
 
@@ -95,6 +97,7 @@ dependencies = [
 name = "driver-framebuffer-virtio"
 version = "0.1.0"
 dependencies = [
+ "hw-virtio",
  "port-framebuffer",
 ]
 
@@ -125,6 +128,10 @@ dependencies = [
  "userspace-rt",
 ]
 
+[[package]]
+name = "hw-virtio"
+version = "0.1.0"
+
 [[package]]
 name = "init"
 version = "0.1.0"