virt: vbox: Use __get_free_pages instead of kmalloc for DMA32 memory

It is not possible to get DMA32 zone memory through kmalloc, causing
the vboxguest driver to malfunction due to getting memory above
4G which the PCI device cannot handle.

This commit changes the kmalloc calls where the 4G limit matters to
using __get_free_pages() fixing vboxguest not working on x86_64 guests
with more then 4G RAM.

Cc: stable@vger.kernel.org
Reported-by: Eloy Coto Pereiro <eloy.coto@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>