using libvirt 4.5 with upstream qemu

[Thanos Makatos]

I'm trying to use QEMU master with libvirt 4.5 and QEMU seems to be hanging
when I try to start a guest.

My environment is a modified CentOS 7.9 installation using libvirt 4.5.0. When
I use a modified version of QEMU 2.12 (reasonably close to the stock CentOS
version) everything works fine. When I try to use a fairly recent version of
QEMU (e.g. v5.2.0-729-g89ff714f4b).

  qemu     118657  1.6  0.0      0     0 ?        Z    14:50   0:00 [qemu-kvm] <defunct>
  qemu     118664  0.0  0.0 207340  3560 ?        Ssl  14:50   0:00 /usr/libexec/qemu-kvm -S -no-user-config -nodefaults -nographic -machine none,accel=kvm:tcg -qmp unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile /var/lib/libvirt/qemu/capabilities.pidfile -daemonize
  qemu     118666  0.0  0.0 275008 13916 ?        Sl   14:50   0:00 /usr/libexec/qemu-kvm -S -no-user-config -nodefaults -nographic -machine none,accel=kvm:tcg -qmp unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile /var/lib/libvirt/qemu/capabilities.pidfile -daemonize

/var/lib/libvirt/qemu/capabilities.pidfile contains the PID of the 2nd QEMU
process and by experimenting I found that by killing the QEMU process NOT in
the PID file the same thing happens again (maybe once more or twice) and then
the guest boots fine.

Does libvirt have some specific QEMU dependency? Is there some compatibility
matrix I might have missed? I understand this may not be a supported
configuration given that I'm not using vanilla libvirt/QEMU, however I'd
appreciate some pointers so I can further debug this. I've trying debugging
this in case there's some obvious error but didn't find anything interesting.

Any ideas?