seccomp: report more useful errors from seccomp

seccomp: report more useful errors from seccomp

Most of the seccomp functions return errnos as a negative return
value. The code is currently ignoring these and reporting a generic
error message for all seccomp failure scenarios making debugging
painful. Report a more precise error from each failed call and include
errno if it is available.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Eduardo Otubo <otubo@redhat.com>