Antiviral effect of antileukemic drugs N4‐Behenoyl‐1‐β‐D‐Arabinofuranosylcytosine (BH‐AC) and 2,2′‐Anhydro‐1‐β‐D‐Arabinofuranosylcytosine (Cyclo‐C) Against Human Cytomegalovirus