exec_close(): return -errno on errors (v2)

All qemu_fclose() callers were already changed to accept any negative
value as error, so we now can change it to return -errno.

When the process exits with a non-zero exit code, we return -EIO to as a
fake errno value.

Changes v1 -> v2:
 - Don't use "//" comments, to make checkpatch.pl happy

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