Reverts PR#777 and adds a new error message for a discretely different case.

lib/vagrant-libvirt/action/create_network_interfaces.rb

@@ -53,7 +53,7 @@ module VagrantPlugins
             else
               free_slot = find_empty(adapters)
               @logger.debug "Adapter not specified so found slot #{free_slot}"
-              raise Errors::InterfaceSlotNotAvailable if free_slot.nil?
+              raise Errors::InterfaceSlotExhausted if free_slot.nil?
             end
 
             # We have slot for interface, fill it with interface configuration.

lib/vagrant-libvirt/errors.rb

@@ -117,6 +117,10 @@ module VagrantPlugins
         error_key(:interface_slot_not_available)
       end
 
+      class InterfaceSlotExhausted < VagrantLibvirtError
+        error_key(:interface_slot_exhausted)
+      end
+
       class RsyncError < VagrantLibvirtError
         error_key(:rsync_error)
       end