These represent xsave-related capabilities of the processor, and KVM may
or may not support them.

Add feature bits so that they are considered by "-cpu ...,enforce", and use
the new feature work instead of calling kvm_arch_get_supported_cpuid.

Bit 3 (XSAVES) is not migratables because it requires saving MSR_IA32_XSS.
Neither KVM nor any commonly available hardware supports it anyway.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>