block: fix overlapping multiwrite requests

block: fix overlapping multiwrite requests

When request A is a strict superset of request B:

  AAAAAAAA
    BBBB

multiwrite_merge() merges them as follows:

  AABBBB

The tail of request A should have been included:

  AABBBBAA

This patch fixes data loss but this code path is probably rare.  Since
guests cannot assume ordering between in-flight requests, few
applications submit overlapping write requests.

Reported-by: Slava Pestov <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Benoit Canet <[email protected]>