$ cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

auto br0
iface br0 inet manual
        bridge_ports enp2s0f2
        bridge_fd 9
        bridge_hello 2
        bridge_maxage 12
        bridge_stp off

$ ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp2s0f2: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast master br0 state DOWN group default qlen 1000
    link/ether 60:45:cb:b3:a0:be brd ff:ff:ff:ff:ff:ff
3: wlp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 3c:a0:67:15:d9:13 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.109/24 brd 192.168.1.255 scope global dynamic wlp3s0
       valid_lft 28015sec preferred_lft 28015sec
24: br0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 60:45:cb:b3:a0:be brd ff:ff:ff:ff:ff:ff

In /etc/libvirt/qemu/Pastel.xml I have:

    <interface type='bridge'>
      <mac address='52:54:00:a0:2e:44'/>
      <source bridge='br0'/>
      <model type='rtl8139'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>