Prefer x2apic mode when running inside a virtual machine.

Prefer x2apic mode when running inside a virtual machine.

Provide a tunable 'machdep.x2apic_desired' to let the administrator override
the default behavior.

Provide a read-only sysctl 'machdep.x2apic' to let the administrator know
whether the kernel is using x2apic or legacy mmio to access local apic.

Tested with Parallels Desktop 8 and bhyve hypervisors.
Also tested running on bare metal Intel Xeon E5-2658.

Obtained from:	NetApp
Discussed with:	jhb, attilio, avg, grehan