lxd/device/proxy: Don't allow NAT mode when used inside projects with networks feature
lxd/device/proxy: Don't allow NAT mode when used inside projects with networks feature

OVN NICs are not reachable from the host.

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

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