multifd: multifd_send_sync_main now returns negative on error

multifd: multifd_send_sync_main now returns negative on error

Even though multifd_send_sync_main() currently emits error_reports, it's
callers don't really check it before continuing.

Change multifd_send_sync_main() to return -1 on error and 0 on success.
Also change all it's callers to make use of this change and possibly fail
earlier.

(This change is important to next patch on  multifd zero copy
implementation, to make it sure an error in zero-copy flush does not go
unnoticed.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220513062836.965425-7-leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>