Backup: split backup creation and creating tar

Backup: split backup creation and creating tar

In preparation for integrating PMG with PBS split the current creation of
a PMG backup into 2 methods:
* create all files in a backup in a target directory
* create a tarball from a backup in a temporary directory

use the changed method in the backup API call.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>