Shares of Virgin Galactic Holdings Inc. are up 3.9% in premarket trades Thursday, after sliding over the last two days.  The stock is down five of the last six days and ended Tuesday’s session down 6.8%, registering its biggest daily percentage decline since Jan. 16, when it fell 7.6%. Last week the space-tourism company completed the penultimate mission of its Unity spacecraft before it halts commercial operations to develop its new Delta-class spacecraft. Related: Virgin Galactic’s stock rises after Tr ...