vbox: fix _displayTakeScreenShotPNGToArray
vbox: fix _displayTakeScreenShotPNGToArray

This function was not implemented for vbox 5+ which removed
TakeScreenShotPNGToArray but provides TakeScreenShotToArray with
BitmapFormat_PNG argument which is the same thing.

crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,datto/libvirt,fabianfreyer/libvirt,taget/libvirt,jfehlig/libvirt,libvirt/libvirt,eskultety/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,datto/libvirt,taget/libvirt,andreabolognani/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,taget/libvirt,olafhering/libvirt,taget/libvirt,datto/libvirt,eskultety/libvirt,jfehlig/libvirt,jardasgit/libvirt,datto/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,libvirt/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,olafhering/libvirt,zippy2/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,jardasgit/libvirt,datto/libvirt,taget/libvirt,fabianfreyer/libvirt