Add a `GPU acceleration` YAML field and a CLI flag

[unsuman]

Assuming that other drivers will support GPU acceleration too in the future, maybe this can be a proper YAML field and a CLI flag.

Can be discussed in a follow-up issue/PR.

Originally posted by @AkihiroSuda in #4137 (comment)

[unsuman]

Now, there is no use of GPUAccel in krunkit because it does not support disabling GPU access inside the VM as far as their documentation goes but we can disable the GPU by removing the --containerd=system arg as a workaround. See #4271
Any thoughts? And should we proceed to add this flag to other drivers now?

[AkihiroSuda]

Let's always enable gpu for krunkit for now