x86/xen: Make sure X2APIC_ENABLE bit of MSR_IA32_APICBASE is not set

Commit d524165c ("x86/apic: Check x2apic early") tests X2APIC_ENABLE
bit of MSR_IA32_APICBASE when CONFIG_X86_X2APIC is off and panics
the kernel when this bit is set.

Xen's PV guests will pass this MSR read to the hypervisor which will
return its version of the MSR, where this bit might be set. Make sure
we clear it before returning MSR value to the caller.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>