Xen: Ignore the retry argument in stop instance

Commit 4ad45119 changed the KVM hypervisor to send multiple shutdown
requests to the monitor, but it didn't change this for the Xen
hypervisor. We simply remove the return on retry model, since we do want
to send multiple shutdown signals for both Xen and KVM (even if the
behaviour is not perfect, they should behave the same).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>