lxc: Fix return values of veth.c functions

lxc: Fix return values of veth.c functions

Previously, the functions in src/lxc/veth.c could sometimes return
positive values on failure rather than -1. This made accurate error
reporting difficult, and led to one failure to catch an error in a
calling function.

This patch makes all the functions in veth.c consistently return 0 on
success, and -1 on failure. It also fixes up the callers to the veth.c
functions where necessary.

Note that this patch may be related to the bug:

  https://bugzilla.redhat.com/show_bug.cgi?id=607496.

It will not fix the bug, but should unveil what happens.

* po/POTFILES.in - add veth.c, which previously had no translatable strings
* src/lxc/lxc_controller.c
* src/lxc/lxc_container.c
* src/lxc/lxc_driver.c    - fixup callers to veth.c, and remove error logs,
                            as they are now done in veth.c
* src/lxc/veth.c - make all functions consistently return -1 on error.
* src/lxc/veth.h - use ATTRIBUTE_NONNULL to protect against NULL args.