Requesting an automated switch to a post-copy migration (using
--timeout-postcopy) without actually enabling post-copy migration (using
--postcopy) doesn't really do anything. Let's make this dependency
explicit to avoid unexpected behavior.
https://bugzilla.redhat.com/show_bug.cgi?id=1455023
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>