If subblock size is large (e.g. 1G) 32 bit math involving it
can overflow. Rather than try to catch all instances of that,
let's tweak block size to 64 bit.

It ripples through UAPI which is an ABI change, but it's not too late to
make it, and it will allow supporting >4Gbyte blocks while might
become necessary down the road.

Fixes: 5f1f79bb ("virtio-mem: Paravirtualized memory hotplug")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>