This helper reverses a list of regions within a [low, high]
span, turning original regions into holes and original
holes into actual regions, covering the whole UINT64_MAX span.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Yanghang Liu <yanghliu@redhat.com>
Reviewed-by: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>