Command line interfaces should use dash, not underscore, as many
keyboard layouts allow that to be typed with fewer shift key presses.

Also, the US spelling of --tunneled gets more google hits than the
UK spelling of --tunnelled.

* tools/virsh.c (opts_migrate): Allow US variant.
(opts_blkdeviotune): Prefer - over _.
* tools/virsh.pod (blkdeviotune): Fix spelling.