hw/arm/virt: Support for virtio-mem-pci

hw/arm/virt: Support for virtio-mem-pci

This supports virtio-mem-pci device on "virt" platform, by simply
following the implementation on x86.

   * This implements the hotplug handlers to support virtio-mem-pci
     device hot-add, while the hot-remove isn't supported as we have
     on x86.

   * The block size is 512MB on ARM64 instead of 128MB on x86.

   * It has been passing the tests with various combinations like 64KB
     and 4KB page sizes on host and guest, different memory device
     backends like normal, transparent huge page and HugeTLB, plus
     migration.

Co-developed-by: David Hildenbrand <david@redhat.com>
Co-developed-by: Jonathan Cameron <Jonathan.Cameron@huawei.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: 20220111063329.74447-3-gshan@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>