VMM: fixed VERR_IEM_INSTR_NOT_IMPLEMENTED Guru Meditation under certain conditions (5.1 regression; mostly Mac OS X hosts).
VMM: fixed software virtualization on Solaris hosts (5.1.20 regression).
Storage: fixed a potential hang under rare circumstances.
Storage: fixed a potential crash under rare circumstances (asynchronous I/O disabled or during maintenance file operations like merging snapshots).
Linux hosts: make the ALSA backend work again as well as loading the GL libraries on certain hosts (5.1.20 regression).
Linux Additions: fixed mount.vboxsf symlink problem (5.1.20 regression).