pxar: only generate .pxarexclude-cli if there were CLI parameters

pxar: only generate .pxarexclude-cli if there were CLI parameters

previously a .pxarexclude entry in the root of the archive caused the file to
be generated as well, because the patterns are read before calling
generate_directory_file_list and within the function it wasn't possible to
distinguish between a pattern coming from the CLI and a pattern coming from
archive/root/.pxarexclude

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>