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.

sys/amd64/vmm/x86.c

@@ -361,6 +361,12 @@ x86_emulate_cpuid(struct vm *vm, int vcpu_id,
 			break;
 
 		case CPUID_0000_0006:
+			regs[0] = CPUTPM1_ARAT;
+			regs[1] = 0;
+			regs[2] = 0;
+			regs[3] = 0;
+			break;
+
 		case CPUID_0000_000A:
 			/*
 			 * Handle the access, but report 0 for