xend: Balloon down memory to achive enough DMA32 memory for PV guests

xend: Balloon down memory to achive enough DMA32 memory for PV guests
with PCI pass-through to succesfully launch.

If the user hasn't used dom0_mem=3D bootup parameter, the privileged
domain usurps all of the memory. During launch of PV guests with PCI
pass-through we ratchet down the memory for the privileged domain to
the required memory for the PV guest. However, for PV guests with PCI
pass-through we do not take into account that the PV guest is going to
swap its SWIOTLB memory for DMA32 memory - in fact, swap 64MB of
it. This patch balloon's down the privileged domain so that there are
64MB of DMA32 memory available.

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>