Comment 3 for bug 1797715

    <interface type='network'>
      <mac address='52:54:00:41:51:b4'/>
      <source network='default'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

$ virsh net-info default
Name: default
UUID: 5db32803-fd1e-4876-98d8-da194f535c17
Active: yes
Persistent: yes
Autostart: yes
Bridge: virbr0

$ virsh net-dumpxml default
<network>
  <name>default</name>
  <uuid>5db32803-fd1e-4876-98d8-da194f535c17</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='virbr0' stp='on' delay='0'/>
  <mac address='52:54:00:71:44:4c'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

from the journal:
Okt 15 17:18:06 juergen kernel: virbr0: port 2(vnet0) entered blocking state
Okt 15 17:18:06 juergen kernel: virbr0: port 2(vnet0) entered disabled state
Okt 15 17:18:06 juergen kernel: device vnet0 entered promiscuous mode
Okt 15 17:18:06 juergen kernel: virbr0: port 2(vnet0) entered blocking state
Okt 15 17:18:06 juergen kernel: virbr0: port 2(vnet0) entered listening state
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7302] manager: (vnet0): new Tun device (/org/freedesktop/NetworkManager/Devices/8)
Okt 15 17:18:06 juergen systemd-udevd[7327]: link_config: autonegotiation is unset or enabled, the speed and duplex are not writable.
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7453] devices added (path: /sys/devices/virtual/net/vnet0, iface: vnet0)
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7454] device added (path: /sys/devices/virtual/net/vnet0, iface: vnet0): no ifupdown configuration found.
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7470] device (vnet0): state change: unmanaged -> unavailable (reason 'connection-assumed', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7513] keyfile: add connection in-memory (697f6001-f8b1-4acc-bc69-b562c78a1372,"vnet0")
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7546] device (vnet0): state change: unavailable -> disconnected (reason 'connection-assumed', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7570] device (vnet0): Activation: starting connection 'vnet0' (697f6001-f8b1-4acc-bc69-b562c78a1372)
Okt 15 17:18:06 juergen libvirtd[1706]: 2018-10-15 15:18:06.757+0000: 1776: error : virNetDevSendEthtoolIoctl:3071 : ethtool ioctl error: No such device
Okt 15 17:18:06 juergen libvirtd[1706]: 2018-10-15 15:18:06.758+0000: 1776: error : virNetDevSendEthtoolIoctl:3071 : ethtool ioctl error: No such device
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7576] device (vnet0): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen libvirtd[1706]: 2018-10-15 15:18:06.758+0000: 1776: error : virNetDevSendEthtoolIoctl:3071 : ethtool ioctl error: No such device
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7588] device (vnet0): state change: prepare -> config (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7595] device (vnet0): state change: config -> ip-config (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7600] device (virbr0): bridge port vnet0 was attached
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7600] device (vnet0): Activation: connection 'vnet0' enslaved, continuing activation
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7605] device (vnet0): state change: ip-config -> ip-check (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7622] device (vnet0): state change: ip-check -> secondaries (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7627] device (vnet0): state change: secondaries -> activated (reason 'none', sys-iface-state: 'external')
Okt 15 17:18:06 juergen NetworkManager[1653]: <info> [1539616686.7787] device (vnet0): Activation: successful, device activated.

the virbr0.status file stays 'empty', the virbr0.macs file contains:
[
  {
    "domain": "win7",
    "macs": [
      "52:54:00:41:51:b4"
    ]
  }
]

hth,
Jürgen