OpenStack - Virtual Machine stuck in task state 'powering-on'

Today for the second time I had to search for the solution in http://dachary.org/?p=2939. Last time, I seemed to have selected the right keywords but it took much more effort today.

I just needed to reset task_state in instances table of the nova database:

mysql -e "update instances set task_state = NULL \
           where deleted = 0 and hostname = 'master'" nova

The problem may be caused by error in nova compute starting up as I use virtual machine manager as well.

Virtual machine manager creates images in /var/lib/libvirt/images which are readable by root only.

For some reason, nova-compute looks in this directory as well and gets errors like:

Stderr: "qemu-img: Could not open '/var/lib/libvirt/images/fedora20.qcow2': Permission denied\n"

I manually change the permissions to readable by all to overcome the problem.