save-cmd.py: disallow backup set names containing "/".

There's no way to refer to them via the VFS, because the VFS (as
expected) treats all forward slashes as path separators.

Before this change, it was possible to create backup sets with names
like "x/y/z" that were inaccessible via ls, restore, etc.

Thanks to Laura Morrissey <laura.morrissey@gmail.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>