security_dac: Limit usage of virSecurityDACSetOwnershipInternal
    
    This function should really be called only when we want to change
    ownership of a file (or disk source). Lets switch to calling a
    wrapper function which will eventually record the current owner
    of the file and call virSecurityDACSetOwnershipInternal
    subsequently.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>