virt-aa-helper: Fix return value of add_file_path

virt-aa-helper: Fix return value of add_file_path

Commit a8853344994a7c6aaca882a5e949ab5536821ab5 added this
function and wrapped vah_add_file in it. vah_add_file may
return -1, 0, 1. It returns 1 in case the call to valid_path
detects a restricted file. The original code treated a return
value != 0 as error. The refactored code treats a return
value < 0 as error. This triggers segfault in virt-aa-helper
and breaks virt-aa-helper-test for the restricted file tests.

Make sure that add_file_path returns -1 on error.