core: export vm - perform space checks only for relevant disks

Currently the space check on export vm will be done for the image disks,
including shareable and snapshot disks that aren't part of the export.
In this patch the check is changed so that the needed space would be
checked only for the exported disks.

Change-Id: I8d7b8eae635ac88af5c3e1ffbe08be7fcdf9da72
Signed-off-by: Liron Aravot <laravot@redhat.com>