core, restapi: add GetVmOvfConfigurationQuery

This patch adds the GetVmOvfConfigurationBySnapshotQuery query which
returns the Ovf configuration of sinapshot (if present).

Usage:
GET Request with HTTP header: All-Content:True , URL:  /api/vms/GUID/snapshots/GUID

Change-Id: I46340c4461b57a4c314fb50ca9c19ac5fd08a451
Signed-off-by: Liron Aravot <laravot@redhat.com>