hooking: Update file() to open()

file() is getting deprecated.

Change-Id: Ib906b4962b719cf7d263e89592abbac8315374d5
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26583
Reviewed-by: Dan Kenigsberg <danken@redhat.com>