avoid unecessary try for over optimization

avoid unecessary try for over optimization

That was lots of code and hash map touching for the case where one
avoided a extra stat, which result probably was in the page cache
anyway, for the case that a backup has a comment.

A case which is rather  be unlikely - comments are normally done for
the occasional explicit backup (e.g., before major upgrade, before a
configuration change in that guest, ...), at least not worth some
relatively complicated effort making that sub harder to read and
maintain.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>