vzdump: move remaining guest include logic to single method

vzdump: move remaining guest include logic to single method

The `guest include` logic handling `all` and `exclude` parameters was in
the `PVE::VZDump->exec_backup()` method. Moving this logic into the
`get_included_guests` method allows us to simplify and generalize it.

This helps to make the overall logic easier to test and develop other
features around vzdump backup jobs.

The method now returns a hash with node names as keys mapped to arrays
of VMIDs on these nodes that are included in the vzdump job.

The VZDump API call to create a new backup is adapted to use the new
method to create the list of local VMIDs and the skiplist.

Permission checks are kept where they are to be able to handle missing
permissions according to the current context. The old behavior to die
on a backup job when the user is missing the permission to a guest and
the job is not an 'all' or 'exclude' job is kept.

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