[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/kernel/xfile.cc

@@ -263,8 +263,8 @@ object_ref<XFile> XFile::Restore(KernelState* kernel_state,
   vfs::File* vfs_file = nullptr;
   vfs::FileAction action;
   auto res = kernel_state->file_system()->OpenFile(
-      abs_path, vfs::FileDisposition::kOpen, access, is_directory, &vfs_file,
-      &action);
+      nullptr, abs_path, vfs::FileDisposition::kOpen, access, is_directory,
+      &vfs_file, &action);
   if (XFAILED(res)) {
     XELOGE("Failed to open XFile: error {:08X}", res);
     return object_ref<XFile>(file);