sdn: register MAC in IPAM if not found

sdn: register MAC in IPAM if not found

if inside add_dhcp_mapping, which is called at VM or LCX start, we do
not find an IP in IPAM, register the MAC.

This is very useful as a fallback if for some reason an IP mapping was
deleted or there is a bug somewhere that does not register an IP.

This acts more like DHCP to allocate an IP on demand.

In order to properly register the IP, the VMID and hostname is required
as a parameter.

Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>