fixed handling of nil volumes

lib/vagrant-libvirt/action/package_domain.rb

@@ -32,6 +32,8 @@ module VagrantPlugins
           )
           domain = env[:machine].provider.driver.connection.servers.get(env[:machine].id.to_s)
           root_disk = domain.volumes.select do |x|
+            !x.nil?
+          end.select do |x|
             x.name == libvirt_domain.name + '.img'
           end.first
           raise Errors::NoDomainVolume if root_disk.nil?