build-qemu: Use the virtio tablet instead of USB for x86

Switch the x86 QEMU configuration to use virtio-tablet-pci instead of
usb-tablet. This allows testing the new virtio input device driver and
does not need 'usb start' to work.

The virtio tablet provides the same functionality as USB tablet but
integrates with the VirtIO subsystem that U-Boot already supports.

Series-to: concept
Series-cc: heinrich

Signed-off-by: Simon Glass <sjg@chromium.org>
Series-links: 1:35

scripts/build-qemu

@@ -278,7 +278,8 @@ class BuildQemu:
             qemu_cmd.extend(['-display', 'default,show-cursor=on'])
         elif self.args.arch == 'x86':
             qemu_cmd.extend(['-device', 'qemu-xhci'])
-            qemu_cmd.extend(['-device', 'usb-kbd', '-device', 'usb-tablet'])
+            qemu_cmd.extend(['-device', 'usb-kbd'])
+            qemu_cmd.extend(['-device', 'virtio-tablet-pci'])
             qemu_cmd.extend(['-display', 'default,show-cursor=on'])
         if not any(item.startswith('-serial') for item in self.qemu_extra):
             qemu_cmd.extend(['-serial', 'mon:stdio'])