Attempt to find the lxc-version command using sudo

... if lxc-version is not found in the efault PATH

Fixes #195

lib/vagrant-lxc/provider.rb

@@ -26,10 +26,14 @@ module Vagrant
       end
 
       def ensure_lxc_installed!
-        unless system("which lxc-version > /dev/null")
+        unless system("which lxc-version > /dev/null 2>&1")
+          begin
+            sudo_wrapper.run("which", "lxc-version")
+          rescue Vagrant::LXC::Errors::ExecuteError
             raise Errors::LxcNotInstalled
           end
         end
+      end
 
       # If the machine ID changed, then we need to rebuild our underlying
       # container.