virtio-mem: Correct default THP size for ARM64

virtio-mem: Correct default THP size for ARM64

The default block size is same as to the THP size, which is either
retrieved from "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
or hardcoded to 2MB. There are flaws in both mechanisms and this
intends to fix them up.

  * When "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size" is
    used to getting the THP size, 32MB and 512MB are valid values
    when we have 16KB and 64KB page size on ARM64.

  * When the hardcoded THP size is used, 2MB, 32MB and 512MB are
    valid values when we have 4KB, 16KB and 64KB page sizes on
    ARM64.

Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-id: 20220111063329.74447-2-gshan@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>