vz: add validation callbacks
    
    This patch fixes a bug which occurs when we check a bus and unit number
    for a new attached disk. We should do this check in ValidadionCallback,
    not in PostParse callback. Because in PostParse we have not initialized
    disk->info.addr.drive struct yet.
    Move part of code from domainPostParseCallback to domainValidateCallback
    and part from devicesPostParseCallback to deviceValidateCallback.
    PostParse callbacks are for modification data.
    ValidateCallbacks are only for checks.