lxd/network/driver/physical: Fix ipv4.routes validation
lxd/network/driver/physical: Fix ipv4.routes validation

Fixes regression from 632c588fe8cf90af11637b307c94b3f4d8d9c599

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

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