fix #545: interfaces: allow arbitrary bridge names in network config

fix #545: interfaces: allow arbitrary bridge names in network config

Similar to other interface types, we can detect a bridge by the
presence of its bridge_ports attribute, rather than solely relying on
the "vmbr" ifname prefix heuristic. For OVS bridges we need to examine
the OVS type instead.

The check needs to be moved up since other prefixes could
theoretically be included in a bridge name and then would otherwise
get picked up wrongly.

Also added a warning for interfaces named vmbrX that are not bridges
to catch possible misconfigurations.

Originally-by: Jillian Morgan <jillian.morgan@primordial.ca>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>