migrate: Use boxed qapi for migrate-set-parameters

Now that QAPI makes it easy to pass a struct around, we don't
have to declare as many parameters or local variables.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>