partially fix #3056: namespace vzdump tmpdir with vmid

partially fix #3056: namespace vzdump tmpdir with vmid

this fixes an issue where a rogue running backup would upload the vm
config of a later backup in a backup job

instead now that directory gets deleted and the config is not
available anymore

we cannot really keep those directories around until the end of the
backup job, since we temporarily save ct contents there, which could get
large very fast

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>