My OS X virtual machine hangs on boot with the gray screen in Parallels Desktop 10
I attempt to run OS X 10.8.0-4 virtual machine on a Mac with a relatively new hardware (e.g on MacPro 6,1, MacBookPro 11,x and other Haswell hardware) or OS X 10.8.5 and 10.9.0 virtual machine on the new MacPro (MacPro 6,1).
Parallels Desktop Development team is aware of this issue and is working on a permanent fix.
Open your virtual machine's Configuration -> Hardware -> Boot Order
Add the following boot flag into the Boot flags field: