My old x86 server broke so I moved to a new x86 motherboard. It booted but the networking stopped working. eth0 was no longer in use I guess. This is my new config and it seems to work for the host and VM guests. I had to physically log in at the machine to get networking working before I could SSH in and diagnose further.
Code:
auto lo
iface lo inet loopback
# iface enp2s0 inet manual
# my new ethernet is enp1s0
iface enp1s0 inet manual
auto enp1s0
#iface enp1s0
auto vmbr0
iface vmbr0 inet static
address your.ip.here.0/24
gateway gateway.ip.here.4
# fix vmbr0 connecting to the right network
# bridge-ports enp2s0
bridge-ports enp1s0
bridge-stp off
bridge-fd 0
source /etc/network/interfaces.d/*