core: ProcessOvfUpdate - skip vms/templates without ovf

When ovf hasn't been generated yet for a vm/template, avoid trying to
add it's ovf to the created tar.

Change-Id: I4de180c59e7e82b380bfc12f5d72f9b7c2a4f468
Bug-Url: https://bugzilla.redhat.com/1129258
Signed-off-by: Liron Aravot <laravot@redhat.com>