xen/hvm: correct reporting of modified memory under physmap during migration

When global_log_dirty is enabled VRAM modification tracking never
worked correctly. The address that is passed to xen_hvm_modified_memory()
is not the effective PFN but RAM block address which is not the same
for VRAM.

We need to make a translation for this address into PFN using
physmap. Since there is no way to access physmap properly inside
xen_hvm_modified_memory() let's make it a global structure.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>