Extend archive_info to include filename and logfilename

Extend archive_info to include filename and logfilename

Only expect the logfilename if the archive has a standard name.
This also gives a mechanism to get an untainted filename.

archive_info can take either a volume ID or a path as it's
currently implemented. This is useful for vzdump when there
is no storage (i.e. for 'vzdump --dumpdir'). Add a test case for this.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>