You should realize that when you boot a virtual machine, it sees the virtual disk as if it was a physical device and, as I understand from your description, the system you boot resides on the disk. So look from the point of view of your normal system: You've got a bigger disk but have the old-sized partition on it. Certainly, you must resize it. But not after booting into that very system (that is, not from the disk) - just as you should never resize your partition, from which your system booted.
So the solution is to download a bootable CD iso image like SystemRescueCD or Gparted live. Add it to your virtual machine (just in the menu you showed in your picture - select the "SATA Controller", click "Add CD/DVD Device" icon and then, on the right, browse for your iso file) and set to boot from that instead of the disk image (but do not remove the image of course). Once you've booted, run
gparted and resize the partition. Shutdown the machine, remove the iso from it and boot back to your virtual disk :)