fix #3385: api: network: check for used ports before writing

fix #3385: api: network: check for used ports before writing

Currently the check for used ports for bonds and bridges happens
while rendering '/etc/network/interfaces.new' in PVE::Inotify
(pve-common).
However at that stage the new/updated interface is already merged
with the old settings, making it impossible to indicate where a NIC
is currently used.

The code is adapted from the renderer in
PVE::Inotify::__write_etc_network_interfaces.

Tested on a virtual PVE instance.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>