virt: vbox: Add vbg_req_free() helper function

This is a preparation patch for fixing issues on x86_64 virtual-machines
with more then 4G of RAM, atm we pass __GFP_DMA32 to kmalloc, but kmalloc
does not honor that, so we need to switch to get_pages, which means we
will not be able to use kfree to free memory allocated with vbg_alloc_req.

While at it also remove a comment on a vbg_alloc_req call which talks
about Windows (inherited from the vbox upstream cross-platform code).

Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>