tools/xend: move assert to exception block

tools/xend: move assert to exception block

The two assert in restore trigger sometimes after hundreds of
migrations. If they trigger the destination host will not destroy the
newly created, yet empty guest. After a second migration attempt to this
host there will be two guets with the same name and uuid. This situation
is poorly handled by the xm tools.
With this change the empty guest will be destroyed.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Matt Wilson <msw@amazon.com>