For now, disable x2APIC mode when Xen is detected, even if CPU
declares support for it. Newer versions of Xen works fine with x2APIC
code, but e.g. Xen 4.2 delivers GPF on the LAPIC MSR write, despite
x2APIC mode being known to hypervisor.

Discussed with: royger
Sponsored by: The FreeBSD Foundation