qemu: switch QEMU capabilities to use virFileCache
    
    The switch contains considerable amount of changes:
    
      virQEMUCapsRememberCached() is removed because this is now handled
      by virFileCacheSave().
    
      virQEMUCapsInitCached() is removed because this is now handled by
      virFileCacheLoad().
    
      virQEMUCapsNewForBinary() is split into two functions,
      virQEMUCapsNewData() which creates new data if there is nothing
      cached and virQEMUCapsLoadFile() which loads the cached data.
      This is now handled by virFileCacheNewData().
    
      virQEMUCapsCacheValidate() is removed because this is now handled by
      virFileCacheValidate().
    
      virQEMUCapsCacheFree() is removed because it's no longer required.
    
      Add virCapsPtr into virQEMUCapsCachePriv because for each call of
      virFileCacheLookup*() we need to use current virCapsPtr.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>