Procedure

This procedure uses virt-manager GUI interface. I'm sure there is a way to do this from the command line as well.

Create a shared device to be used by two VMs
Open virt-manager

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-1.png

Double click on one of the VMs which you want to add the shared device on and go to View->Details

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-2.png

Select: Add Hardware on the bottom left
Select: Select or create custom storage

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-3.png

Select: Manage

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-4.png

Select: "+"

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-5.png

Give the volume a name
Select "raw" from the "format" dropdown
Give it a size
Hit "Finish"
Back in the "Add New Virtual Hardware" window, set "Bus Type" to "SCSI"
Hit "Finish"
In the "Detailed" view of the VM, under the new storage device, select "Shareable"

Lustre Network (LNet) > Setting up a Failover Pair with virsh/virt-manager > vm-6.png

On the second VM, go through the same steps to add a storage device, but select the existing shared device you just created and make it shareable as well
Start the VMs
You should now see a "/dev/sda" (or similar) which is the shared device
To setup OST failover for example, do the following from one of the VMs using the shared device

# on VM-1
mkfs.lustre --ost --fsname=lustrewt --servicenode=192.168.122.114@tcp --servicenode=192.168.122.110@tcp --reformat --index=0 --mgsnode=192.168.122.117@tcp /dev/sda
mount -t lustre /dev/sda /mnt/ost/

# to go through a failover do
# on VM-1
umount /mnt/ost

# on VM-2
mount -t lustre /dev/sda /mnt/ost/