src: fix generation of default resource partition

src: fix generation of default resource partition

Now that resource structure can have appid as well we need to adapt code
that creates default resource partition if not provided by user.
Otherwise starting a VM with appid defined would fail with following
error:

    error: unsupported configuration: Resource partition '(null)' must start with '/'

Fixes: 38b5f4faabccf681439d99e5394954c6ef7a5a40
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>