core: correct path for snapshot links

The links to the relevant snapshot objects should be under the snapshot
object and not under the vm object to be consistent with the displayed
path.

Change-Id: I223b0cef14b887c1e1e3cff6f06b01fd3ac050c9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077678
Signed-off-by: Liron Aravot <laravot@redhat.com>