fix #2609 api: backupinfo: add non job specific endpoint

fix #2609 api: backupinfo: add non job specific endpoint

Adds a new api endpoint at cluster/backupinfo for cluster wide backup
stuff. This is necessary because cluster/backup expects a backup job ID
at the next level and thus other endpoints are hard to impossible to
implement under that hierarchy.

The only api endpoint available for now is the `not_backed_up` which
returns a list of all guests which are not covered by any backup job.

The top level index endpoint is left unsused for now to be available for
a more generic summary endpoint in the future.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>