Add disk name for disk config

plugins/kernel_v2/config/disk.rb

@@ -112,8 +112,9 @@ module VagrantPlugins
         @size = nil if @size == UNSET_VALUE
         @primary = true if @primary == UNSET_VALUE
 
-        # generate name instead of nil if unset_value
-        @name = nil if @name == UNSET_VALUE
+        # Give the disk a default name if unset
+        # TODO: Name not required if primray?
+        @name = "vagrant_#{@type.to_s}_#{@id.split("-").last}" if @name == UNSET_VALUE
 
         @config = nil if @config == UNSET_VALUE
       end

plugins/kernel_v2/config/vm.rb

@@ -791,6 +791,8 @@ module VagrantPlugins
           errors << "There is more than one disk defined for guest '#{machine.name}'. Please pick a `primary` disk."
         end
 
+        # TODO: Check for duplicate disk names?
+
         @__disks.each do |d|
           error = d.validate(machine)
           errors.concat error if !error.empty?