virsh: Elide backslash-newline in batch mode
    
    The previous patch made it possible to split multiple commands by
    adding newline, but not to split a long single command. The sequence
    backslash-newline was being used as if it were a quoted newline
    character, rather than completely elided the way the shell does.
    
    Again, add more tests, although this time it seems more like I am
    suffering from a leaning-toothpick syndrome with all the \.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>