start: Fix print_top_failing_dir for /var/lib/lxc

start: Fix print_top_failing_dir for /var/lib/lxc

In the case where /var/lib/lxc itself was not accessible,
print_top_failing_dir would fail to print the error message.

This fixes it and also change the initial access check for X_OK instead
of R_OK (to match what we actually need and print_top_failing_dir's own
check).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>