ui: IPRef field: allow all valid IPs, even if not in the store

ui: IPRef field: allow all valid IPs, even if not in the store

since commit 1ccb53ecdbd69a0e4c57a6d861da1918d3625e08 from widget
toolkit[0] we enforce by default that an entered value needs to be in
the backing data store, else the field is marked invalid.

But, for the firewall source/destination IP selector we do not care
if it's in the store, which is only loaded for convenience and to
allow selecting defined aliases/ipsets

[0]: https://git.proxmox.com/?p=proxmox-widget-toolkit.git;a=commit;h=1ccb53ecdbd69a0e4c57a6d861da1918d3625e08

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>