virsh: Use consistent naming for blkdeviotune options
    
    All options started with underscores, but we switched them to dashes
    later on, making the style consistent.  The latest addition, however,
    did not respect that, so let's change that as well.  It is tempting to
    just change the name instead of adding alias, especially since nobody
    ever used it, which we know thanks to the fact that it didn't work.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>