Use /proc/device-tree for reading AVF DT

Although /proc/device-tree is symlink to /sys/firmware/devicetree/base,
/proc/device-tree is the stable API but the absolute path may be
changed in the future.

Bug: 322465386
Test: atest CustomPvmfwHostTestCases
Change-Id: I0d55918b6ec4563135c596649ccfd4bd84284963
diff --git a/virtualizationmanager/src/debug_config.rs b/virtualizationmanager/src/debug_config.rs
index 003a7d4..e2b657a 100644
--- a/virtualizationmanager/src/debug_config.rs
+++ b/virtualizationmanager/src/debug_config.rs
@@ -45,7 +45,7 @@
         // unwrap() is safe for to_str() because node_path and prop_name were &str.
         PathBuf::from(
             [
-                "/sys/firmware/devicetree/base",
+                "/proc/device-tree",
                 self.node_path.to_str().unwrap(),
                 "/",
                 self.prop_name.to_str().unwrap(),