In the last 5 years, virtualization software has been massively adopted by companies as a mean to reduce costs, achieve instant scalability, and possibly better their security through isolation. Recent numbers indicate that 78 percent of companies have their production servers virtualized, and 20 percent of them actually only rely on virtualized servers. At the same time, security auditing of such software poses unique challenges, in particular when it comes to dynamic testing. In this paper, we describe a methodology for the security assessment of virtualization software based on switching the CPU mode to virtual 8086 mode in order to get access to the (possibly virtualized) hardware, that aims at being both generic (applicable to both x86 and x64 architectures) and extremely large in terms of code coverage. We have implemented this technology under the form of a dynamic testing tool which has proved to be very efficient in finding bugs in virtualization software.