emulators/virtualbox-ose-additions{,-nox11}: Fix vboxvfs bugs

This commit fixes vboxvfs (shared foler) kernel module bugs:

• fix open file leaks on creating files that causes file descriptor leaks on host;
• fix halfway lookups.

PR: 245865
MFH: 2025Q1
(cherry picked from commit 1a2220ab88eed87566727fef85f4cb7fb0d95965)