virtio: Fix running on machines with memory above 0xffffffff

virtio: Fix running on machines with memory above 0xffffffff

We want to allocate a contiguous memory block anywhere in memory, but
expressed this as having to be between 0 and 0xffffffff. This limits us
on 64-bit machines, and outright breaks on machines where memory is
mapped above that address range.

Allow the full address range to be used for this allocation.

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