extend Virtualbox convenince methods

In the vein of #2426, I too can never remember syntax, and end up
hunting for either an exisiting Vagrantfile or starting to dig up
details.

plugins/providers/virtualbox/config.rb

@@ -84,6 +84,14 @@ module VagrantPlugins
         customize("pre-boot", ["modifyvm", :id, "--memory", size.to_s])
       end
 
+      # Shortcut for setting CPU count for the virtual machine.
+      # Calls #customize internally.
+      #
+      # @param count [Integer, String] the count of CPUs
+      def cpus=(count)
+        customize("pre-boot", ["modifyvm", :id, "--cpus", count.to_i])
+      end
+
       # This is the hook that is called to finalize the object before it
       # is put into use.
       def finalize!

website/docs/source/v2/virtualbox/configuration.html.md

@@ -62,11 +62,11 @@ more than 50% would be used on your own host machine. Some details:
 * Multiple `customize` directives can be used. They will be executed in the
   order given.
 
-There is also a shortcut for specifying the memory size (MB) for the virtual
-machine:
+There are some convenience shortcuts for memory and CPU settings:
 
 ```ruby
 config.vm.provider "virtualbox" do |v|
   v.memory = 1024
+  v.cpus = 2
 end
 ```