CA-113543: Don't spam syslog for benign request failures.

This was most likely a debug print added when the NBD funcionality was
implemented. tapdisk either way complains about failed requests
elsewhere.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #59 on xapi-project/blktap