Accept all MAC changes from the VM

[PlagueCZ]

Fixes #768

In a previous PR #714 I changed the behavior of MAC address discovery for the VM.

The idea was to only update the MAC once, so prevent unnecessary memory copy.

I knew that the user can change a MAC manually in the VM, but that was really not a concern.

What I missed though is the fact, that iPXE and systemd both use different MAC address:

1. There is the MAC address of the VF (can be seen on host)
2. iPXE starts the VM and requests DHCP - with the VF's original MAC
3. iPXE downloads kernel, initramfs and squashfs and starts the image
4. systemd starts DHCP but due to MACAddressPolicy=persistent in /usr/lib/systemd/network/99-default.link it generates a new MAC
5. dpservice refuses to update it because of PR Feature/vf mac change func #714
6. kernel drops all incoming traffic due to MAC mismatch

I was able to make this happen on many host machine reboots. After applying this PR I am unable to do it

[byteocean]

Thanks for explaining. LGTM.