Shares of Citrix Systems (CTXS) and VMware (VMW) were down for a second day following Amazon.com’s (AMZN) announcement that its Amazon Web Services unit will enter the desktop virtualization market to boost its cloud computing business. VMware’s virtualization software is widely used in corporate data centers to increase the flexibility and capabilities of computer servers, but it’s also moving onto desktops. Citrix has focused its virtualization