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

@@ -26,8 +26,12 @@ module Vagrant
       end
 
       def ensure_lxc_installed!
-        unless system("which lxc-version > /dev/null")
-          raise Errors::LxcNotInstalled
+        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