x86: tsc: punish VirtualBox with low priority for TSC

[kevans]

Summary

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

[kib]

From what I understand in the timecounter selection logic, tc with negative tc_quality can be selected with sysctl, but not by newly added tunable. If yes, it might make sense to set quality to zero, not a negative.

[emaste]

LGTM with or without @kib's suggestion

[kevans]

In D29132#652389, @kib wrote:

From what I understand in the timecounter selection logic, tc with negative tc_quality can be selected with sysctl, but not by newly added tunable. If yes, it might make sense to set quality to zero, not a negative.

Right, that looks correct to me as well. I don't see any reason we should ignore this if it's specified in the new tunable, and if we really don't have any with a quality higher than 0 then we do have a functional system with accurate enough timekeeping to keep ntpd happy. I'll switch it to 0 pre-commit.