Improve veth error cases logging

The previous error message made it look like the host interface name was
the concatenation of both the host and guest interface names.

Closes #374

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
fixed typo in comment

Signed-off-by: Shuai Zhang <zs.broccoli@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>