So, investigated this into an apparmor difference between 4.17 and 4.18.
This is due to commit 338d0be437ef10e247a35aed83dbab182cf406a2
("apparmor: fix ptrace read check").

libvirtd here is using only trace, and not read. The patch below for
libvirtd apparmor policy fixes it for me.

--- /etc/apparmor.d/usr.sbin.libvirtd   2018-08-23 14:52:04.574252908 -0300
+++ ../usr.sbin.libvirtd        2018-08-23 14:51:46.773728841 -0300
@@ -50,10 +50,10 @@
   # for --p2p migrations
   unix (send, receive) type=stream addr=none peer=(label=unconfined addr=none),
 
-  ptrace (trace) peer=unconfined,
-  ptrace (trace) peer=/usr/sbin/libvirtd,
-  ptrace (trace) peer=/usr/sbin/dnsmasq,
-  ptrace (trace) peer=libvirt-*,
+  ptrace (read,trace) peer=unconfined,
+  ptrace (read,trace) peer=/usr/sbin/libvirtd,
+  ptrace (read,trace) peer=/usr/sbin/dnsmasq,
+  ptrace (read,trace) peer=libvirt-*,
 
   signal (send) peer=/usr/sbin/dnsmasq,
   signal (read, send) peer=libvirt-*,

** Also affects: libvirt (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: libvirt (Ubuntu Cosmic)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Cosmic)
   Importance: High
     Assignee: Thadeu Lima de Souza Cascardo (cascardo)
       Status: In Progress

** Changed in: libvirt (Ubuntu Cosmic)
   Importance: Undecided => Critical

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https://bugs.launchpad.net/bugs/1788603

Title:
  libvirt fails with failure to open mount namespace

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