core: handle ovf empty UUID for quota

Due to latest changes, quota field cannot accept empty UUID.
prior to that change, a VM could have been exported with an empty
UUID to OVF file.
Altering the ovf reader to ignore empty UUID for quota.

Change-Id: Ibcd0b29861a43bcb77acd7c3e0bae58d7b27b8f4
Bug-Url: https://bugzilla.redhat.com/1025773
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>