core: Register all the OVF disks to Storage

Manage all the OVF_STORE disks with one list,
so all those disks will be registered as floating shareable disks in the Storage
and will also be used for getting the most updateable OVF_STORE disk to
be retrieved entities from.

Change-Id: I9fc6666a64efa16968935cfceb10e19b8a4e2eef
Bug-Url: https://bugzilla.redhat.com/1138114
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>