Hello Adrian,
I could not reproduce the same problem you reported, but found two other problems when trying to run VirtualBox in Qemu:
First, 'atapi_drv' does not run well on newer Qemu versions (I used Qemu 2.2.0), so I changed your run script to use the 'ahci' driver instead (see attachment).
Then I tried to run TinyCore Linux in the VM (with both the .vbox and the .iso file in the root directory of the ext2 image) and when the Linux kernel started to boot, VirtualBox reported 'invalid gueststate' errors and the kernel boot process never finished.
So, even if you can find out why your VirtualBox complains about the 'w7.vbox' file, I would not have too high hopes of getting your VM running in VirtualBox on Qemu (or with AMD SVM hardware virtualization in general) without some prior work on the VirtualBox port. So far we have used and tested VirtualBox mainly on Intel hardware. Would that be an option for you?
Regards, Christian