QEMUFileCloseFunc: add return value documentation (v2)

QEMUFileCloseFunc: add return value documentation (v2)

qemu_fclose() and QEMUFile->close will return -errno on error, and any
positive value on success.

We need the positive non-zero success values because
migration-exec.c:exec_close() relies on non-zero return values to get
the process exit code.

Changes v1 -> v2:
 - Cosmetic spelling change on comment text

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