Howto

1. Download Yosemite from the App Store
2. Open Terminal.app
3. Install iesd, to customize OS X InstallESD:
   gem install iesd
4. Turn install image into base system:
   iesd -i "/Applications/Install OS X Yosemite.app" -o yosemite.dmg -t BaseSystem
5. Convert into UDSP (sparse image) format:
   hdiutil convert yosemite.dmg -format UDSP -o yosemite.sparseimage
6. Mount the InstallESD …
   hdiutil mount "/Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg"
7. … as well as the sparse image:
   hdiutil mount yosemite.sparseimage
8. Copy base system into sparse image:
   cp "/Volumes/OS X Install ESD/BaseSystem."* "/Volumes/OS X Base System/"
9. Unmound InstallESD …
   hdiutil unmount "/Volumes/OS X Install ESD/"
10. … as well as the sparse image:
    hdiutil unmount "/Volumes/OS X Base System/"
11. Unmount both mounted disks via diskutil:
    • diskutil unmountDisk $(diskutil list | grep "OS X Base System" -B 4 | head -1)
      
    • diskutil unmountDisk $(diskutil list | grep "OS X Install ESD" -B 4 | head -1)
12. Convert back to UDZO format (compressed image):
    hdiutil convert yosemite.sparseimage -format UDZO -o yosemitefixed.dmg
13. Add yosemitefixed.dmg as a live cd in virtual box
14. Change the chipset of your virtual machine to “PIIX3“
15. Start your VM, open Disk Utility within installer and create a new HFS+ partition on the virtual disk
16. Install it!