nwfilter: Save config to disk if we generated a UUID

nwfilter: Save config to disk if we generated a UUID

libvirt-daemon-config-nwfilter will put a bunch of xml configs
into /etc/libvirt/nwfilter. These configs don't hardcode a UUID
and depends on libvirt to generate one. However the generated UUID
is never saved to disk, unless the user manually calls Define.

This makes daemon reload quite noisy with many errors like:

error : virNWFilterObjAssignDef:3101 : operation failed: filter 'allow-incoming-ipv4' already exists with uuid 50def3b5-48d6-46a3-b005-cc22df4e5c5c

Because a new UUID is generated every time the config is read from
disk, so libvirt constantly thinks it's finding a new nwfilter.

Detect if we generated a UUID when the config file is loaded; if so,
resave the new contents to disk to ensure the UUID is persisteny.

This is similar to what was done in commit a47ae7c0 with virtual
networks and generated MAC addresses