Template is saved as single archive of the whole VM directory. Preserve
backup directory structure regardless of its content - in this case it
means we need "." archive (with template directory content) placed in
"vm-tempates/<template-name>/" backup directory. This allows restore
process to select right files to restore regardless of VM type.
Also some major cleanups: Reduce some more code duplication
(verify_hmac, simplify backup_restore_prepare). Rename
backup_dir/backup_tmpdir variables to better match its purpose. Rename
backup_do_copy back to backup_do. Require QubesVm object (instead of VM
name) as appvm param.
