virtio-mem: Make sure "usable_region_size" is always multiples of the block size

The spec states:
  "The device MUST set addr, region_size, usable_region_size, plugged_size,
   requested_size to multiples of block_size."

With block sizes > 256MB, we currently wouldn't guarantee that for the
usable_region_size.

Note that we cannot exceed the region_size, as we already enforce the
alignment there properly.

Fixes: 910b2576 ("virtio-mem: Paravirtualized memory hot(un)plug")
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20201008083029.9504-3-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>