CLOUDSTACK-6755: [OVS] Can't create more than 7 GRE tunnel networks in
xen cluster

XenServer does not create a bridge automatically when VIF from domU is connected
to internal network. So there is logic to force bridge creation by
creating VIF in dom0 connected to GRE tunnel network. But there is no
logic to delete the VIF after bridge gets created. So this fix ensure
VIF is delted when atleast there is one domU VIF connected to the
network.

(cherry picked from commit 9e4e62466a8c3d00ab5381d7ebfeb70466192a55)