Public bug reported:

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

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1875139

Title:
  Domain fails to start when 'readonly' device not writable

Status in QEMU:
  New

Bug description:
  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

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