Domain fails to start when 'readonly' device not writable

This issue is introduced in QEMU 4.2.0 (4.1.0 is working fine)

My root disk is a LVM2 volume thin snapshot that is marked as read-only
But when I try to start the domain (using virt-manager) I get the following error:

Error starting domain: internal error: process exited while connecting to monitor: 2020-04-26T06:55:06.342700Z qemu-system-x86_64: -blockdev {"driver":"host_device","filename":"/dev/vg/vmroot-20200425","aio":"native","node-name":"libvirt-3-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"} The device is not writable: Permission denied

Changing the lvm snapshot to writeable allows me to start the domain.
(Making it changes possible during domain is running)

I don't think QEMU should fail when it can't open a (block) device when the read-only option is set.
(why is write access needed?)

Reproduce steps:
* Create LVM read-only volume (I don't think any data is needed)
* Create domain with read-only volume as block device
* Try to start the domain