Retrospectively adding qemu guest agent channel to volume backed instance

[Craig McIntyre]

Description
===========

Having deployed a number of instances in our openstack environment it was discovered that we need to have qemu guest agent functionality enabled, which was not baked into the image these instances were deployed from.

Instances are cinder volume backed, not ephemeral.

I raised the question in the #openstack-nova irc channel if this was possible. The only feasible option seemed to be a rebuild. With this in mind, and given that the instances are volume backed I was looking at
 potential alternative options to work around this.

Environment
===========
DISTRIB_ID="OSA"
DISTRIB_RELEASE="18.1.3"
DISTRIB_CODENAME="Rocky"
DISTRIB_DESCRIPTION="OpenStack-Ansible"

Nova v18.1.1

Hypervisor: Libvirt (libvirt 4.0.0) + KVM (qemu-kvm version 1:2.11+dfsg-1ubuntu7.23)

Cinder v13.0.3 backed volumes