Fix preopened directory mapping to virtual root

This commit fixes the bug where preopening `.` causes the WASI runner to
map it to the virtual root, causing `path_open` to fail.

fixes wasmerio#4361

lib/wasix/src/runners/wasi_common.rs

@@ -49,7 +49,7 @@ impl CommonWasiOptions {
 
         if self.mapped_dirs.iter().all(|m| m.guest != ".") {
             // The user hasn't mounted "." to anything, so let's map it to "/"
-            builder.add_map_dir(".", "/")?;
+            builder.add_map_dir(".", "/mnt/host")?;
         }
 
         builder.set_fs(Box::new(fs));