MFC r280134:
Report ARAT (APIC-Timer-always-running) feature for virtual CPU.

This makes FreeBSD guest to not avoid using LAPIC timer, preferring HPET
due to worries about non-existing for virtual CPUs deep sleep states.

Benchmarks of usleep(1) on guest and host show such extra latencies:

• 51us for virtual HPET,
• 22us for virtual LAPIC timer,
• 22us for host HPET and
• 3us for host LAPIC timer.