core: Verify that date is initialized in OVF_STORE disk

Verify that the last_update date is initizlied in the OVF_STORE disk to prevent NPE.
If the last_update date is not initialized in the disk we don't count
this disk to be retrieved, unregistered entities, from, since it is not
initilized with entities.

Change-Id: I8e59086ce7242d7aaec10b57b6775ab968c9fe1c
Bug-Url: https://bugzilla.redhat.com/1130976
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>