xen/pvshim: keep track of used PFN ranges

xen/pvshim: keep track of used PFN ranges

Simple infrastructure to keep track of PFN space usage, so that we can
use unpopulated PFNs to map special pages like shared info and grant
table.

As rangeset depends on malloc being ready so hypervisor_setup is
introduced for things that can be initialised late in the process.

Note that the PFN is marked as reserved at least up to 4GiB (or more
if the guest has more memory). This is not a perfect solution but
avoids using the MMIO hole below 4GiB. Ideally the shim (L1) should
have a way to ask the underlying Xen (L0) which memory regions are
populated, unpopulated, or MMIO space.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>