qemu: switch QEMU capabilities to use virFileCache

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>