Extending existing Virtual Box/VMware partition

Create new virtual disk with dynamic space DiskNew.vdi with total space
VBoxManage clonehd DiskOld.vmdk DiskNew.vdi --existing
Remove DiskOld.vmdk in Settings
Add DiskNew.vdi
Boot into Windows
Disk Management compmgmt.msc
Right click C:
Click Extend Volume
Select the extra unpartitioned space and click Finish