cpu: x86: Add internal CPUID features support and KVM feature bits

cpu: x86: Add internal CPUID features support and KVM feature bits

Some of the emulator features are presented in the <features> element in
the domain XML although they are virtual CPUID feature bits when
presented to the guest. To avoid confusing the users with these
features, as they are not configurable via the <cpu> element, this patch
adds an internal array where those can be stored privately instead of
exposing them in the XML.

Additionaly KVM feature bits are added as example usage of this code.