* adt-virt-lxc: Don't try to set up a shared downtmp with unprivileged (user) containers. This is full of permission errors due to the mapped UIDs. * tests/adt-run LxcRunner: Use $ADT_TEST_LXC as unprivileged user container if it exists. * adt-buildvm-ubuntu-cloud: Check at the beginning if QEMU command is available and exit with proper error message if not. (LP: #1460066) * If auxverb fails with exit code 126, 127, 254, or 255, consider this a failure of auxverb/the testbed itself, not of the program run under it. (As specified in doc/README.virtualisation-server.rst) (LP: #1462540)