tests/migrate: update to use argparse and run spicy by default
tests/migrate: update to use argparse and run spicy by default

Most helpful change is that it prints help if you do --help.
It can take --image parameter, and --client parameter, defaults to spicy.

Incidentally, spicy switch host seems broken - if you run with --client spicec
(assuming spicec is in the PATH), it works fine, but using the default spicy (or
--client spicy) it fails after a single switch.

dezelin/spice,mathslinux/spice,SPICE/spice,dezelin/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,elmarco/spice,mathslinux/spice,fgouget/spice,Fantu/spice,ya-mouse/spice,ya-mouse/spice,elmarco/spice,alon/spice-server,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,SPICE/spice,SPICE/spice,mathslinux/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,fgouget/spice,mathslinux/spice,mathslinux/spice,dezelin/spice,alon/spice-server,Fantu/spice,elmarco/spice,Fantu/spice,fgouget/spice,alon/spice-server,elmarco/spice,alon/spice-server,Fantu/spice,dezelin/spice