I have installed and setup KVM server on an Ubuntu Linux. I downloaded various .qcow2 cloud images. How do I setup public ssh keys for downloaded CentOS/FreeBSD/Ubuntu cloud images using virt-sysprep?

You need to use a command called virt-sysprep that lets you reset or unconfigure virtual machines in preparation for cloning them.

Step 1: Install virt-sysprep

Type the following apt-get command/apt command to install virt-sysprep on a Debian or Ubuntu Linux:
$ sudo apt install libguestfs-tools
If you are using a CentOS/RHEL/Oracle/Scientific Linux, type the following yum command:
$ sudo yum install libguestfs-tools
If you are using a Fedora Linux, type the following dnf command to install the same:
$ sudo dnf install libguestfs-tools

Step 2: Download cloud image in .qcow2 format

You can grab cloud images from the following sites (grab the file ending with .qcow2/.qcow2.xz extensions):

1. CentOS 7
2. CentOS 6.x
3. Debian 8.x
4. Debian 9.x
5. Fedora 26
6. Ubuntu 16.04 LTS
7. FreeBSD 11.x
8. openSUSE/SLES
9. AWS Linux
10. RHEL 7 (subscription only)
11. RHEL 6(subscription only)
12. SLES(subscription only)

For demo purpose I am downloading and using CentOS 7 image using wget command:
$ wget https://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2.xz
Use xz command for decompression:
$ xz -v -d CentOS-7-x86_64-GenericCloud.qcow2.xz

Step 3: Setup/inject an ssh keys

To inject an ssh key so the given “USER” will be able to log in over ssh without supplying a password. The “USER” must exist already in the guest. For CentOS 7 user name is centos:
$ sudo virt-sysprep -a CentOS-7-x86_64-GenericCloud.qcow2 \
--ssh-inject centos:file:/home/vivek/.ssh/id_rsa.pub
Where,

• --ssh-inject centos:file:/home/vivek/.ssh/id_rsa.pub : Read the ssh key from id_rsa.pub.

It is also possible to create a new user named vivek and add ssh-key as follows:
$ sudo virt-sysprep -a CentOS-7-x86_64-GenericCloud.qcow2 \
--run-command 'useradd vivek' \
--ssh-inject vivek:file:/home/vivek/.ssh/id_rsa.pub
Sample outputs:

Step 4: Launch a new VM using CentOS-7-x86_64-GenericCloud-1503.qcow2 image

The syntax is:
$ virt-install --import \
--name centos7-vm1 \
--memory 1024 \
--vcpus 2 \
--cpu host \
--disk path=/var/lib/libvirt/images/centos7.qcow2,size=10,bus=virtio,format=qcow \
--os-type=linux \
--os-variant=centos7.0 \
--graphics spice \
--noautoconsole \
--disk /home/vivek/modifyisoimages/CentOS-7-x86_64-GenericCloud.qcow2

Step 5: Test it with ssh

To find out your vm’s IP address run:
$ virsh net-list
$ virsh net-dhcp-leases default
To login using ssh command:
$ ssh vivek@vms-ip-address-here

Other options to set ssh key for your cloud images

• Use uvt-kvm on a Ubuntu Linux to setup ssh-keys for cloud image.
• Use cloudinit to setup ssh keys
• Also read virt-sysprep man page.