Disable TSC usage inside SMP VM environments. On my VMware ESXi 4.1
environment with a core i5-2500K, operation in this mode causes timeouts
from the mpt driver. Switching to the ACPI-fast timer resolves this issue.
Switching the VM back to single CPU mode also works, which is why I have
not disabled the TSC in that mode.

I did not test with KVM or other VM environments, but I am being cautious
and assuming that the TSC is not reliable in SMP mode there as well.

Reviewed by: kib
Approved by: re (kib)
MFC after: Not applicable, the timecounter code is new for 9.x