libxl: close fd's in parent when spawning qdisk

libxl: close fd's in parent when spawning qdisk

Coverity points out that these remain open in the parent upon
success, which is a resource leak.

To fix this rejig the exit paths such that success and error cases
both close the two fds, this means adjusting the callback to only
happen for the error case and it also makes sense to rename the label
from "error" to just "out".

Compile tested only.

CID: 1130518 (null) and 1130517 (logfile_w).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>