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: NSlava Pestov <sviatoslav.pestov@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>