iommu: remove the share_p2m operation

iommu: remove the share_p2m operation

Sharing of HAP tables is now VT-d specific so the operation is never defined
for AMD IOMMU any more. There's also no need to pro-actively set vtd.pgd_maddr
when using shared EPT as it is straightforward to simply define a helper
function to return the appropriate value in the shared and non-shared cases.

NOTE: This patch also modifies unmap_vtd_domain_page() to take a const
      pointer since the only thing it calls, unmap_domain_page(), also takes
      a const pointer.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>