Add public_address provider capability

Used by vagrant-share plugin to determine local guest address for
forwarding via ngrok.

lib/vagrant-libvirt/cap/public_address.rb

@@ -0,0 +1,16 @@
+module VagrantPlugins
+  module ProviderLibvirt
+    module Cap
+      class PublicAddress
+        def self.public_address(machine)
+          # This does not need to be a globally routable address, it
+          # only needs to be accessible from the machine running
+          # Vagrant.
+          ssh_info = machine.ssh_info
+          return nil if !ssh_info
+          ssh_info[:host]
+        end
+      end
+    end
+  end
+end

lib/vagrant-libvirt/plugin.rb

@@ -39,6 +39,11 @@ module VagrantPlugins
         Cap::NicMacAddresses
       end
 
+      provider_capability(:libvirt, :public_address) do
+        require_relative 'cap/public_address'
+        Cap::PublicAddress
+      end
+
       # lower priority than nfs or rsync
       # https://github.com/vagrant-libvirt/vagrant-libvirt/pull/170
       synced_folder('9p', 4) do