drivers/char: mark DMA buffers as reserved for the XHCI

drivers/char: mark DMA buffers as reserved for the XHCI

The important part is to include those buffers in IOMMU page table
relevant for the USB controller. Otherwise, DbC will stop working as
soon as IOMMU is enabled, regardless of to which domain device assigned
(be it xen or dom0).
If the device is passed through to dom0 or other domain (see later
patches), that domain will effectively have access to those buffers too.
It does give such domain yet another way to DoS the system (as is the
case when having PCI device assigned already), but also possibly steal
the console ring content. Thus, such domain should be a trusted one.
In any case, prevent anything else being placed on those pages by adding
artificial padding.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>