Add autobackup functionality to CLI

Adds autobackup (integrated, managed VM backups with automatic remote
filesystem mounting, included backup expiry/removal and automatic
full/incremental selection, independent from the manual "pvc vm backup"
commands) to the CLI client.

This is a bit of a large command to handle only inside the CLI client,
but this was chosen as it's the only real place for it aside from an
external script.

There are several major restrictions on this command, mainly that it
must be run from the primary coordinator using the "local" connection,
and that it must be run as "root".

The command is designed to run in a cron/systemd timer installed by
pvc-ansible when the appropriate group_vars are enabled, and otherwise
not touched.

debian/pvc-client-cli.install

@@ -0,0 +1 @@
+client-cli/autobackup.sample.yaml usr/share/pvc