virsh migrate: Fix positional parameters

virsh migrate: Fix positional parameters

Thanks to our smart option parser which automatically assigns positional
parameters the following (previously working) command fails:

    virsh migrate test qemu+ssh://1.2.3.4/system tcp://1.2.3.4/
    error: invalid argument: Unsupported compression method
    'tcp://1.2.3.4/'

We need to make sure new options are added at the end of the list rather
than where they logically belong.

Reported by Brian Rak.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>