[Kernel/VFS] Specify root entry to open from + cleanup.

[VFS] Allow specifying root entry to open from with OpenFile.
[Kernel] NtCreateFile now opens from root entry when available instead
         of needlessly building a full path and resolving from that.
[VFS] Reduce code duplication by adding Entry::ResolvePath.
[VFS] Remove ResolveBasePath to avoid multiple calls to find_base_guest_path.

src/xenia/vfs/entry.h

@@ -97,6 +97,7 @@ class Entry {
   bool is_read_only() const;
 
   Entry* GetChild(const std::string_view name);
+  Entry* ResolvePath(const std::string_view path);
 
   const std::vector<std::unique_ptr<Entry>>& children() const {
     return children_;