hv_xen: Report memory used by hypervisor

- Report memory used by hypervisor (“mem_hv” as per resource model
  design document, “xmem” in htools)
- Also report number of CPUs available to Dom0
- Some other, small changes

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>