add ivshmem device to config

add ivshmem device to config

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>