qemu: capabilities: force update if the microcode version does not match
    
    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.
    
    Therefore, place microcode version in the virQEMUCaps struct and
    XML, and rebuild the cache if the versions do not match.
    
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>