core: Fix error cause when exporting a VM with sharable disks

Fix error caused when exporting a VM that contains sharable disks along
with regular disks, after the export an update on the VM copy in SPM
is executed and trying to get the image info of the sharable images also
and fails since they do not exist since they are not exported in the first
place

Change-Id: Ibd85529186e29f602496764687794b9075a7f84f
Bug-Url: https://bugzilla.redhat.com/885452
Signed-off-by: Tal Nisan <tnisan@redhat.com>