I did it! There is something like a little groove at the side - at the connection between black and white parts - wherein I slotted a screwdriver, twisted it, and the light cover pulled off. Eureka! – Lukman Dec 17 '16 at 12:16
That appears to be intended to pry out with a screwdriver or similar flat tool at the indents a little past 6 o'clock and a little past 12 o'clock as pictured.
I've never seen a "plug" that completely covers the socket, but you can get a two-prong adapter like this that will fill in the Edison socket and only leave the two prongs opened.
Best Answer
Answered by OP in comments: