vhost: Allow adjoining regions

vhost: Allow adjoining regions

My rework of section adding combines overlapping or adjoining regions,
but checks they're actually the same underlying RAM block.
Fix the case where two blocks adjoin but don't overlap; that new region
should get added (but not combined), but my previous patch was disallowing it.

Fixes: c1ece84e7c9
Reported-by: Alex Williamson <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Tested-by: Alex Williamson <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>