[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/devices/host_path_device.cc

@@ -53,21 +53,8 @@ Entry* HostPathDevice::ResolvePath(const std::string_view path) {
   // The filesystem will have stripped our prefix off already, so the path will
   // be in the form:
   // some\PATH.foo
-
   XELOGFS("HostPathDevice::ResolvePath({})", path);
-
-  // Walk the path, one separator at a time.
-  auto entry = root_entry_.get();
-  auto path_parts = xe::utf8::split_path(path);
-  for (auto& part : path_parts) {
-    entry = entry->GetChild(part);
-    if (!entry) {
-      // Not found.
-      return nullptr;
-    }
-  }
-
-  return entry;
+  return root_entry_->ResolvePath(path);
 }
 
 void HostPathDevice::PopulateEntry(HostPathEntry* parent_entry) {