[Schema transformer] Retrun a correct set from VirtualNetworkST.delete

The methode VirtualNetworkST shoud always return a set() event if the
virtual network was already deleted.

Change-Id: Ib34bc255c3e63e70ecfa8e8c73f2786c60bd3845
Signed-off-by: Édouard Thuleau <edouard.thuleau@cloudwatt.com>
(cherry picked from commit 73d665713dda74f8f052459f812d557e1d164dbc)