nodeinfo: Check for SYSFS_INFINIBAND_DIR before open
Commit id 'ac3ed2085' causes 'virsh nodedev-list --cap net' to fail
on any system without SYSFS_INFINIBAND_DIR (/sys/class/infiniband).
Rather than assume it's there and fail on the attempt to open the
non-existent directory, check if it's there - if not, return
success and move on. Also fix caller to check < 0 upon return.
As reported by Suren Hajyan <shajyan@redhat.com> from run of unit tests