use directIO flags when dd'ing template

This makes sure dom0 in xenserver doesn't get hammered
when copying templates. It doesn't make sense to use
the cache of dom0 as the template does not fit in
memory. The directIO flags prevent it from trying.

(cherry picked from commit 4e1527e87aaaa87d14d3c7d3a6782b80cbf36a8c)