fix lxc-attach returned error

fix lxc-attach returned error

When we try to attach to a container belonging to another user than us,
the command fails as expected but the return code is wrong, so we have
an "unknown error" instead of "permission denied".

The culprit is:

-                     strerror(command.answer.ret));
+                     strerror(-command.answer.ret));

The rest of the code is indentation without code impact.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Michel Normand <normand@fr.ibm.com>