xen: introduce a per-arch scratch mapping ranges

The current approach is to create foreign mappings in any unpopulated address
on the memory map. This however can cause issues, as late-loaded drivers could
then found their MMIO region has been stolen to be used as foreign mapping
scratch space (due to the Xen drivers having started first).

Add a local resource manager to the xenpv bus driver, architectures can add
suitable resources to that resource manager at boot in order to be used in
preference of the generic MMIO resource allocator.

No functional change, as the introduced scratch mapping range is not yet
populated.

Sponsored by: Cloud Software Group
MFC after: 1 week
Reviewed by: Elliott Mitchell <ehem+freebsd@m5p.com>
Differential revision: https://reviews.freebsd.org/D46122