core: export vm - removal of unneeded for loop

All the disks that are part of the export are already being iterated
over, their id's can be added during that loop.
Furthermore, there's no need to include id's that aren't part of the
export (of shareable/snapshot disks).

Change-Id: I8f6c3b2ae192674e7edc86840253d6f8ef02d5c0
Signed-off-by: Liron Aravot <laravot@redhat.com>