Let's try to detect the actual THP size and use it as default block size
(unless the page size of the backend indicates that THP don't apply).
Always allow to set a block size of 1 MiB, but warn if the configured block
size is smaller than the default. Handle large block sizes better, avoiding
a virtio-spec violation and optimizing address auto-detection.
For existing setups (x86-64), the default block size won't change (was, and
will be 2 MiB on anonymous memory). For existing x86-64 setups, the address
auto-detection won't change in relevant setups (esp., anonymous memory
and hugetlbfs with 2 MiB pages and no manual configuration of the block
size). I don't see the need for compatibility handling (especially, as
virtio-mem is still not considered production-ready).
Most of this is a preparation for future architectures, using hugetlbfs
to full extend, and using manually configured, larger block sizes
(relevant for vfio in the future).