…e the arguments in the Vbox v[56].* case

As they say, they have a special requirement for limiting network masks to
2^16 allocatable IPs only:

	// For IPv6, the max size will be limited to 65536
	// This is due to the allocator keeping track of all the
	// allocated IP's in a bitmap. This will keep the size of
	// the bitmap to 64k.

Ref: https://github.com/tkestack/tke/blob/v1.9.2/pkg/util/ipallocator/allocator.go#L271-L274

But here there in the VirtualBox CPI is no reason for such a limitation. We
just need the correct calculation of mask length.

With VirtualBox v7 on macOS we can create networks of 'hostonlynets' with a
new CLI experience that allows any name for the created network.

We leverage this new possibility, while still retaining the former feature
with networks of 'hostonlyif' type, when the name of a non-existing and
to-be-created network could possibly be guessed in advance, and if correctly
guessed, then everything was fine.