core: don't write payload data on export

when creating OVF for export or snapshot,
the payload data should not be part of it,
as it may contain sensitive data and may be malformed for the xml.

Change-Id: I662aff50ee5cf7174a135a004081c48c4e1ab5b1
Bug-Url: https://bugzilla.redhat.com/997910
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>