If a command fails with a sense that scsi_sense_buf_to_errno converts to
ECANCELED/EAGAIN/ENOTCONN or with a unit attention, scsi_req_complete is
called twice. This caused a crash.
Reported-by: Wangguang <wang.guangA@h3c.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>