xen: mask the MTRR feature from the cpuid

We don't want Linux to think that the cpu supports MTRRs when running
under Xen because MTRR operations could only be performed through
hypercalls.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>