There are two things here, 

(1) Mel already debugged this above: "Cannot load certificate ... & key ..." for tls-proxy.  The "Error while reading file" (how "helpful") seems like something that is coming GnuTLS or whichever library is being used by Ubuntu.

(2) Warnings about "VMX" flag -- these can be ignored, but let's find the root cause as it is seems very bizarre, because:

  - There is no '-cpu' parameter in the offending guest[*] QEMU 
    command-line, requesting for 'vmx'
  - Current upstream QEMU does _not_ support VMX for TCG (plain emulation)

So the second problem (which are warnings only) seem to be somehow specific to Ubuntu -- did they change something in their QEMU package?


[*] instance-00000002
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2019-03-08 02:23:56.064+0000: starting up libvirt version: 4.0.0, package: 1ubuntu8.6 (Christian Ehrhardt <email address hidden> Fri, 09 Nov 2018 07:42:01 +0100), qemu version: 2.11.1(Debian 1:2.11+dfsg-1ubuntu7.10), hostname: ubuntu-bionic-rax-iad-0003574603
LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin QEMU_AUDIO_DRV=none /usr/bin/qemu-system-x86_64 -name guest=instance-00000002,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-2-instance-00000002/master-key.aes -machine pc-i440fx-bionic,accel=tcg,usb=off,dump-guest-core=off -m 64 -realtime mlock=off -smp 1,sockets=1,cores=1,threads=1 -uuid ae55dba3-ea1c-43f1-a537-1ef9d6c55d40 -smbios 'type=1,manufacturer=OpenStack Foundation,product=OpenStack Nova,version=18.1.0,serial=ae55dba3-ea1c-43f1-a537-1ef9d6c55d40,uuid=ae55dba3-ea1c-43f1-a537-1ef9d6c55d40,family=Virtual Machine' -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-2-instance-00000002/monitor.sock,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot strict=on -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive file=/opt/stack/data/nova/instances/ae55dba3-ea1c-43f1-a537-1ef9d6c55d40/disk,format=qcow2,if=none,id=drive-virtio-disk0,cache=none -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -add-fd set=0,fd=29 -chardev pty,id=charserial0,logfile=/dev/fdset/0,logappend=on -device isa-serial,chardev=charserial0,id=serial0 -vnc 127.0.0.1:0,tls,x509verify=/etc/pki/libvirt-vnc -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -msg timestamp=on
2019-03-08T02:23:56.124629Z qemu-system-x86_64: -chardev pty,id=charserial0,logfile=/dev/fdset/0,logappend=on: char device redirected to /dev/pts/0 (label charserial0)
2019-03-08T02:23:56.135964Z qemu-system-x86_64: warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
2019-03-08T02:23:56.169501Z qemu-system-x86_64: -vnc 127.0.0.1:0,tls,x509verify=/etc/pki/libvirt-vnc: Failed to start VNC server: Cannot load certificate '/etc/pki/libvirt-vnc/server-cert.pem' & key '/etc/pki/libvirt-vnc/server-key.pem': Error while reading file.
2019-03-08 02:23:56.174+0000: shutting down, reason=failed
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