virtio-gpud: Add offset method to XferToHost2d

This will hopefully avoid confusion if someone in the future changes the
XferToHost2d in flush to pass a smaller GpuRect. Without adjusting
offset this would cause glitches.

graphics/virtio-gpud/src/main.rs

@@ -392,7 +392,7 @@ pub struct XferToHost2d {
 }
 
 impl XferToHost2d {
-    pub fn new(resource_id: ResourceId, rect: GpuRect) -> Self {
+    pub fn new(resource_id: ResourceId, rect: GpuRect, offset: u64) -> Self {
         Self {
             header: ControlHeader {
                 ty: VolatileCell::new(CommandTy::TransferToHost2d),
@@ -400,8 +400,8 @@ impl XferToHost2d {
             },
 
             rect,
+            offset,
             resource_id,
-            offset: 0,
             padding: 0,
         }
     }

graphics/virtio-gpud/src/scheme.rs

@@ -193,6 +193,7 @@ impl<'a> Display<'a> {
                 width: self.width,
                 height: self.height,
             },
+            0,
         ))?;
         let header = self.send_request(req).await?;
         assert_eq!(header.ty.get(), CommandTy::RespOkNodata);