core: rename method in ExportVmCommand

Rename the method endDiskRelatedActions to be less obscure and reflect
what it actually does i.e. populating the VM disks & devices data

Change-Id: I468fc1875d8f6045dba376beec2636a144a17efe
Related to Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=922475
Signed-off-by: Liron Aravot <laravot@redhat.com>