selftests/ublk: fix garbage output in foreground mode

selftests/ublk: fix garbage output in foreground mode

Initialize _evtfd to -1 in struct dev_ctx to prevent garbage output
when running kublk in foreground mode. Without this, _evtfd is
zero-initialized to 0 (stdin), and ublk_send_dev_event() writes
binary data to stdin which appears as garbage on the terminal.

Also fix debug message format string.

Fixes: 6aecda00b7d1 ("selftests: ublk: add kernel selftests for ublk")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>