conf: include x86 microcode version in virsh capabilities
    
    A microcode update can cause the CPUID bits to change; an example
    from the past was the update that disabled TSX on several Haswell and
    Broadwell machines.
    
    In order to track the x86 microcode version in the QEMU capabilities,
    we have to fetch it and store it in the host CPU.  This also makes the
    version visible in "virsh capabilities", which is a nice side effect.
    
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>