lxd/device/nic/bridged: Validates network is type bridge
lxd/device/nic/bridged: Validates network is type bridge

Now that different NIC types support the "network" key.

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

stgraber/lxd,Skarlso/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,Skarlso/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,mjeanson/lxd,Skarlso/lxd,lxc/lxd,mjeanson/lxd,atxwebs/lxd,mjeanson/lxd