xen: changes to facilitate "xl" support (xen 4.1)

- Copy the xl config file, in case there's any
- Start instances by config file, not name (also xm compatible)
- Start paused domains with -p and not --paused (also xm compatible)
- Add a fixme for migration (changes are not xm compatible)

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>