Add Hyper-V Dynamic Memory Protocol driver (hv-balloon) hot-add support

Add Hyper-V Dynamic Memory Protocol driver (hv-balloon) hot-add support

One of advantages of using this protocol over ACPI-based PC DIMM hotplug is
that it allows hot-adding memory in much smaller granularity because the
ACPI DIMM slot limit does not apply.

In order to enable this functionality a new memory backend needs to be
created and provided to the driver via the "memdev" parameter.

This can be achieved by, for example, adding
"-object memory-backend-ram,id=mem1,size=32G" to the QEMU command line and
then instantiating the driver with "memdev=mem1" parameter.

The device will try to use multiple memslots to cover the memory backend in
order to reduce the size of metadata for the not-yet-hot-added part of the
memory backend.

Co-developed-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>