autopkgtest-virt-qemu+rocm: Fix multi-GPU pass-through

The current, naive approach to passing in multiple GPUs into a single VM, namely assigning each GPU to its own PCI root bridge, did not work on Argo and Lyra.

This could have multiple reasons:

• root bridge setup could be misconfigured
• maybe all GPUs need to be assigned to a single root bridge
• maybe all GPUs on the same card (Argo and Lyra have cards with two GPUs each) need to be assigned to the same bridge
• etc.