config: net: avoid duplicate ipam entries on nic update

config: net: avoid duplicate ipam entries on nic update

Updating the NIC of a VM when the following conditions were met:
* VM is turned off
* NIC is on a bridge that uses automatic dhcp
* Leave bridge unchanged

led to duplicate IPAM entries for the same network device.

This is due to the fact that the add_next_free_cidr always ran on
applying pending network changes.

Now we only add a new ipam entry if either:
* the value of the bridge or mac address changed
* the network device has been newly added

This way no duplicate IPAM entries should get created.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>