x86: tsc: deprioritize TSC on VirtualBox

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 84eaf2c, applying it only to VirtualBox and
increasing the quality to 0. Negative qualities can never be chosen and
cannot be chosen with the tunable recently added. If we do not have a
timecounter with a higher quality than 0, then TSC does at least leave
the system mostly usable.

PR:		253087
Reviewed by:	emaste, kib
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29132

sys/x86/x86/tsc.c

@@ -503,6 +503,14 @@ test_tsc(int adj_max_count)
 
 	if ((!smp_tsc && !tsc_is_invariant))
 		return (-100);
+	/*
+	 * Misbehavior of TSC under VirtualBox has been observed.  In
+	 * particular, threads doing small (~1 second) sleeps may miss their
+	 * wakeup and hang around in sleep state, causing hangs on shutdown.
+	 */
+	if (vm_guest == VM_GUEST_VBOX)
+		return (0);
+
 	size = (mp_maxid + 1) * 3;
 	data = malloc(sizeof(*data) * size * N, M_TEMP, M_WAITOK);
 	adj = 0;