iotests: Make 083 less flaky

083 has (at least) two issues:

1. By launching the nbd-fault-injector in background, it may not be
   scheduled until the first grep on its output file is executed.
   However, until then, that file may not have been created yet -- so it
   either does not exist yet (thus making the grep emit an error), or it
   does exist but contains stale data (thus making the rest of the test
   case work connect to a wrong address).
   Fix this by explicitly overwriting the output file before executing
   nbd-fault-injector.

2. The nbd-fault-injector prints things other than "Listening on...".
   It also prints a "Closing connection" message from time to time.  We
   currently invoke sed on the whole file in the hope of it only
   containing the "Listening on..." line yet.  That hope is sometimes
   shattered by the brutal reality of race conditions, so make the sed
   script more robust.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171109203025.27493-5-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
