x86/mm: add HVMOP_get_mem_type hvmop

x86/mm: add HVMOP_get_mem_type hvmop

The balloon driver in the guest frees guest pages and marks them as
mmio. When the kernel crashes and the crash kernel attempts to read the
oldmem via /proc/vmcore a read from ballooned pages will generate 100%
load in dom0 because Xen asks qemu-dm for the page content. Since the
reads come in as 8byte requests each ballooned page is tried 512 times.

Add a new hvmop HVMOP_get_mem_type to return the hvmmem_type_t for the
given pfn. Pages which are neither ram or mmio will be HVMMEM_mmio_dm.
This interface enables the crash kernel to skip ballooned pages.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>
Committed-by: Tim Deegan <Tim.Deegan@citrix.com>