unix_close(): check for close() errors too (v2)

In case close() fails, we want to report the error back.

Changes v1 -> v2:
 - Use braces on if statement to match coding style

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
