Add the testbed and modify parts of the code that require it.

Also modify/create Dockerfiles for csit-shim-arm and csit-sut-arm
Current changes in https://github.com/jlinkes/multivppcache

CURRENT ISSUES REPLICABLE WITH 18588 GERRIT PATCH AND LOCAL WORKAROUNDS:
figure out how to use VPP_DEVICE_IMAGE on different platforms (do we need two of these files or can we make an image with the same name?)

csit-sut-arm: fix CSIT scripts to use VPP_DEVICE_IMAGE in vpp device tests so that the default from ContainerUtils.py:786 is not used

memif tests: ICMP echo Rx timeout in ip4 memif (container) tests

vm tests: create /var/lib/vm/vhost-nested.img
vm tests: fix
-machine pc,accel=kvm,usb=off,mem-merge=off
qemu-system-aarch64: -enable-kvm: unsupported machine type 'pc'
Use -machine help to list supported machines