lxd/network/network/utils: Renames ValidNetworkName to validInterfaceName
lxd/network/network/utils: Renames ValidNetworkName to validInterfaceName

- Clarifies error messages.
- Unexports.
- Removes veth prefix check (as not relevant anymore).

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

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