virsh: add [--domain DOMAIN] option to domxml-to-native DOMAIN COMMAND
    
    The option allows someone to run domain-to-native on already existing
    domain without the need of supplying their XML.  It is basically
    wrapper around 'virsh dumpxml  | virsh domxml-to-native /dev/stdin'.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=835476
    Signed-off-by: Daniel Liu <srwx4096@gmail.com>
    Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>