exec_close(): accept any negative value as qemu_fclose() error

Note that we don't return the unchanged return value back yet, because
we need to change all qemu_fclose() callers to accept any positive value
as success.

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