The problem can be resolved by setting the user and group in the /etc/libvirt/libvirtd.conf configuration file as follows.
user = "nova"
group = "nova"
And also changing the ownership as follows.
chown -R nova:nova /var/cache/libvirt
chown -R nova:nova /var/run/libvirt
chown -R nova:nova /var/lib/libvirt
Hope this will work.