with such a shared memory device, a vm can share data with other
vms or with the host via memory
one of the use cases is looking-glass[1] with pci-passthrough, which copies
the guest fb to the host and you get a high-speed, low-latency
display client for the vm
on vm stop we delete the file again
1: https://looking-glass.hostfission.com/
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>