virQEMUCapsReset: also clear out hostCPUModel
    
    After succesfully reading an outdated caps cache from disk,
    calling virQEMUCapsReset did not properly clear out the calculated
    host CPU model. This lead to a memory leak when the host CPU model
    pointer was overwritten later in virQEMUCapsNewForBinaryInternal.
    
    Introduced by commit 68c70118.