lxd/network/driver/ovn: Adds InstanceDevicePortValidateExternalRoutes function
lxd/network/driver/ovn: Adds InstanceDevicePortValidateExternalRoutes function

To validate a OVN NIC's external routes against an OVN network's uplink routes and project's restricted subnet.

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

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