libvirt-daemon: bridged nets:Cannot get interface MAC on 'vnet%d' for second vm/adapter

[Jakob Bohm <jb-debbugs@wisemo.com>]

Package: libvirt-daemon
Version: 3.0.0-4+deb9u2~bpo8+1
Severity: important

NOTICE: This is for regular bridged networks, not the SR-IOV kind that 
had a similar looking issue on Redhat systems sometime before 3.9.0.?

After upgrading from 3.0.0-4~bpo8+1 to 3.0.0-4+deb9u2~bpo8+1 starting 
most VMs fail to start with the following error (as reported in 
libvirtd.log):

  error : virNetDevSetMAC:248 : Cannot get interface MAC on 'vnet%d': No 
  such device

Specifically, it happens in the following scenarios for bridged 
network interfaces:

  - More than one NIC on the same VM (even on different bridges)
  - Running more than one VM with one NIC each

This is badly breaking, as there is little use in a virtualization host 
that can only run one networked VM at a time.

Is there a way to download 3.0.0-4~bpo8+1 again to unbreak the system, 
or will I have to downgrade all the way back to 1.2.9-9+deb5u5 ?

VM type used: kvm
Bridge configuration: Directly in /etc/network/interfaces
Red Hat Network Manager: Not installed
Example bridge config XML snippet from a now failing VM:

<domain type='kvm'>
  ...
  <os>
    <type arch='x86_64' machine='pc-0.12'>hvm</type>
    <boot dev='hd'/>
  </os>
  ...
  <devices>
    ...
    <interface type='bridge'>
      <mac address='xx:xx:xx:xx:xx:xx'/>
      <source bridge='br0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
      function='0x0'/>
    </interface>
    ...
  </devices>
</domain>

(This VM still starts when no other VM is running, but then I can't 
start any other VM after it).

I have found that I can run these VMs simultaneously by running qemu 
outside libvirt control, so it must be libvirt doing something wrong 
when setting up tap devices for the VMs.

-- System Information:
Debian Release: 8.10
  APT prefers oldstable-updates
  APT policy: (500, 'oldstable-updates'), (500, 'oldstable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.16.0-6-amd64 (SMP w/2 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages libvirt-daemon depends on:
ii  libapparmor1        2.10.95-4~bpo8+2
ii  libaudit1           1:2.4-1+b1
ii  libavahi-client3    0.6.31-5
ii  libavahi-common3    0.6.31-5
ii  libblkid1           2.25.2-6
ii  libc6               2.19-18+deb8u10
ii  libcap-ng0          0.7.4-2
ii  libdbus-1-3         1.8.22-0+deb8u1
ii  libdevmapper1.02.1  2:1.02.90-2.2+deb8u1
ii  libfuse2            2.9.3-15+deb8u2
ii  libgnutls-deb0-28   3.3.8-6+deb8u7
ii  libnetcf1           1:0.2.3-4.1
ii  libnl-3-200         3.2.24-2
ii  libnl-route-3-200   3.2.24-2
ii  libnuma1            2.0.10-1
ii  libparted2          3.2-7
ii  libpcap0.8          1.8.1-3~bpo8+1
ii  libpciaccess0       0.13.2-3+b1
ii  librados2           10.2.5-6~bpo8+1
ii  librbd1             10.2.5-6~bpo8+1
ii  libsasl2-2          2.1.26.dfsg1-13+deb8u1
ii  libselinux1         2.3-2
ii  libssh2-1           1.4.3-4.1+deb8u1
ii  libudev1            230-7~bpo8+2
ii  libvirt0            3.0.0-4+deb9u2~bpo8+1
ii  libxen-4.4          4.4.1-9+deb8u10
ii  libxenstore3.0      4.4.1-9+deb8u10
ii  libxml2             2.9.1+dfsg1-5+deb8u6
ii  libyajl2            2.1.0-2

Versions of packages libvirt-daemon recommends:
ii  libxml2-utils   2.9.1+dfsg1-5+deb8u6
ii  netcat-openbsd  1.105-7
ii  qemu-kvm        1:2.8+dfsg-3~bpo8+1

Versions of packages libvirt-daemon suggests:
ii  libvirt-daemon-system  3.0.0-4+deb9u2~bpo8+1

-- no debconf information