Introduce an alternative hole type using the platorm PCI device BAR

Introduce an alternative hole type using the platorm PCI device BAR

Using a memory hole burns 2M of RAM and is only helpful in the case where
the guest has pass-through devices causing Xen to make accesses to all PCI
BARs uncacheable. In the case where guest-visible devices are all emulated
this will not be the case and so we can save the 2M of RAM by using the
platform PCI device BAR as the hole.

This patch adds the necessary code to do that, defaulted off but enabled
by setting the XENBUS registry parameter DWORD:UseMemoryHole to 0.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>