Adding a disk to a libvirt/kvm vm client

So you have a kvm virtualization infrastructure which you, being lazy like me, manage using libvirt. You then created a vm client with the virtual disk partitioned just right (using LVM or not; pick your poison). But later you realized you needed another disk. Maybe it is because you need to have data encrypted to meet HIPAA or PCI requirements. Maybe you just want to keep your data in a different drive. The point is you need another drive and don't want to/should not resize the current virtual disk associated with the VM.
So, first thing you do is create the new disk. If you are using KVM, chances are you have been using qcow2 disks. Or maybe vmdk, iSCSI, or, like me, lvm. No matter what, you probably know how to create a new disk, so I will assumed you took time to figure out how large it needs to be to fit your needs and created the little bastard. Because, as I mentioned before, I am lazy, I will say we are running libvirt in linux, with a vm client called vmclient and creating LVs to use as virtual disks. So, we need a 10GB virtual disk that we'll call data because we are friends with Captain Obvious.

lvcreate -L 10G -n data vmhost_vg0

creates as we know a 10GB lv as /dev/vmhost_vg0/data. To make it easier on us, we will shut down the vm client. Once that is done (do check it using virsh list --all, will you?), we then run

virsh edit vmclient

Note we could have created a properly configured xml file and fed into the config, but I do forget how to make it properly configured so I prefer to cheat.
When the config file is open, look for the disk entries; they should look like this:

    <disk device="disk" type="file">
      <driver cache="none" io="native" name="qemu" type="raw"/>
      <source file="/dev/vmhost_vg0/vmclient_boot"/>
      <target bus="virtio" dev="vda"/>
      <address bus="0x00" domain="0x0000" function="0x0" slot="0x04" type="pci"/>
    </disk>

which is how our original virtual disk is configured to be used in this vm client. Of course, if you were using qcow, vmdk, or something else, the entry might look a bit different; make a note of how it differs from my example and move on. This is why I said I like to cheat: I can see how the old disk was defined and copy that instead of trying to figure out how to do it.

Now you need to add the new drive. As you guessed from the above, we will do it by copying the above entry and changing it a bit. Now, we do not need to copy everything; we just need enough so virsh knows what we want. It will fill the blanks. So, after we copy the relevant bits below the already defined disk and change the drive name, we would have something like this

    <disk device="disk" type="file">
      <driver cache="none" io="native" name="qemu" type="raw"/>
      <source file="/dev/vmhost_vg0/data"/>
      <target bus="virtio" dev="vda"/>
    </disk>

Save it; it should close without issues. If not, got back and see if you missed something. If it did not bark, use virsh dump xml vmclient to see your handiwork. Mine looks like this:

    <disk device="disk" type="file">
      <driver cache="none" io="native" name="qemu" type="raw"/>
      <source file="/dev/vmhost_vg0/vmclient_boot"/>
      <target bus="virtio" dev="vda"/>
      <address bus="0x00" domain="0x0000" function="0x0" slot="0x04" type="pci"/>
    </disk>
    <disk device="disk" type="file">
      <driver cache="none" io="native" name="qemu" type="raw"/>
      <source file="/dev/vmhost_vg0/data"/>
      <target bus="virtio" dev="vda"/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/%gt
    </disk>

As you can see, it added the pci bus address entry on its own. Now all we need to do is reboot the vmclient, format the new disk on whatever way we want, and start using it.