Move the file descriptor type checking before doing anything with it.
If it's not usable, don't close it as it could be in use by another
part of QEMU, only fail and report an error.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>