The default_tls_x509_verify (and related) parameters in qemu.conf
control whether the TLS servers in QEMU request & verify
certificates from clients. This works as a simple access control
system for QEMU servers by requiring the CA to issue certs to
permitted clients. This use of client certificates is disabled by
default, since it requires extra work to issue client certificates.
Unfortunately the libvirt code was using these configuration
parameters when setting up both TLS clients and servers in QEMU. The
result was that TLS clients for character devices and disk devices
had verification turned off, meaning they would ignore any errors
while validating the server certificate.

A MITM attacker can attack any client connection made by QEMU's
character devices and disk devices which have TLS enabled. The
attacker can send an arbitrary certificate back to the client QEMU
and it will ignore all errors that result during validation.

References:

https://www.redhat.com/archives/libvirt-announce/2017-October/msg00001.html