libfdt: Make Fdt::root{,_mut}() infallible

As we now build the node without calling into libfdt, there's no need
for the Rust functions to -> Result<_> so update the API accordingly.

Test: m pvmfw virtmgr fsfdt librialto libfdtpci
Test: liblibfdt.integration_test libpvmfw.device_assignment.test
Change-Id: I82d146b0db851c72cb3d58516f4cc14fff3f0321
diff --git a/virtualizationmanager/src/dt_overlay.rs b/virtualizationmanager/src/dt_overlay.rs
index b39ba3a..108ed61 100644
--- a/virtualizationmanager/src/dt_overlay.rs
+++ b/virtualizationmanager/src/dt_overlay.rs
@@ -61,8 +61,8 @@
 
     let fdt =
         Fdt::create_empty_tree(buffer).map_err(|e| anyhow!("Failed to create empty Fdt: {e:?}"))?;
-    let root = fdt.root_mut().map_err(|e| anyhow!("Failed to get root node: {e:?}"))?;
-    let mut fragment = root
+    let mut fragment = fdt
+        .root_mut()
         .add_subnode(cstr!("fragment@0"))
         .map_err(|e| anyhow!("Failed to add fragment node: {e:?}"))?;
     fragment