range: add some more functions

range: add some more functions

Add some more functions that will be used in memory-device context.

range_init(): Init using lower bound and size, check for validity
range_init_nofail(): Init using lower bound and size, validity asserted
range_size(): Extract the size of a range
range_overlaps_range(): Check for overlaps of two ranges
range_contains_range(): Check if one range is contained in the other

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181214131043.25071-2-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>