x86: mask out unknown features from Xen's capabilities

x86: mask out unknown features from Xen's capabilities

If Xen doesn't know about a feature, it is unsafe for use and should be
deliberately hidden from Xen's capabilities.

This doesn't make a practical difference yet, but will make a difference
later when the guest featuresets are seeded from the host featureset.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>