core: Introduce a public_address capability so that we can use vagrant share

lib/vagrant-lxc/plugin.rb

@@ -30,6 +30,13 @@ module Vagrant
           SyncedFolder
         end
       end
+
+      if Vagrant::Backports.vagrant_1_5_or_later?
+        provider_capability("lxc", "public_address") do
+          require_relative "provider/cap/public_address"
+          Provider::Cap::PublicAddress
+        end
+      end
     end
   end
 end

lib/vagrant-lxc/provider/cap/public_address.rb

@@ -0,0 +1,17 @@
+module Vagrant
+  module LXC
+    class Provider
+      module Cap
+        module PublicAddress
+          def self.public_address(machine)
+            return nil if machine.state.id != :running
+
+            ssh_info = machine.ssh_info
+            return nil if !ssh_info
+            ssh_info[:host]
+          end
+        end
+      end
+    end
+  end
+end