Misbehavior has been observed with TSC under VirtualBox, where threads
doing small sleeps (~1 second) may miss their wake up and hang around
in a sleep state indefinitely. Switching back to ACPI-fast decidedly
fixes it, so stop using TSC on VirtualBox at least for the time being.
This partially reverts 84eaf2ccc6aa, applying it only to VirtualBox.
MFC after: 3 days