virt: Cart config memory and code style optimization.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds cart config variant name to file mapping.

Allows better tests printing.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Improves list tests function.

Allow use the new params built-in variable which maps file to variant name.

Signed-off-by: Jiří Župka <jzupka@redhat.com>