lxd/network/network/utils: Updates UsedBy to check if the network has any created peers
lxd/network/network/utils: Updates UsedBy to check if the network has any created peers

If it does it shows the target network as being a user of the network.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd