had the same problem:
this fixed it:
NOTE 2: Virtualbox may have problems booting Mac OS X if your computer uses an Intel Haswell processor (i.e. if your computer was built/bought in 2013 or later). If this is this case, you will have to trick Virtualbox into thinking that your processor is actually an older model. To do this, open the Command Prompt in Windows (make sure you are logged into an Administrator account on Windows). You can do this by opening the Start Menu, and typing "command prompt" into the Start Menu search bar. Then, type the following command into the Command Prompt.
cd "C:Program FilesOracleVirtualbox"
This command will change the focus of the Command Prompt to the program folder for Virtualbox (if you installed Virtualbox somewhere different, then change the command to wherever you installed it). Next, type in this command: VBoxManage modifyvm <Name of virtual machine> --cpuidset 00000001 000306a9 00020800 80000201 178bfbff
This command activates "vboxmanage", a command-line program included with Virtualbox that allows you to edit the properties of your virtual machine, including its built-in resolutions. Replace "Name of virtual machine" with the name of your virtual machine (no quotation marks)-- you can find the name of your virtual machine in the left pane of the Virtualbox main window. This command should do the trick.