x86/mm: add HYPERVISOR_memory_op to acquire guest resources

x86/mm: add HYPERVISOR_memory_op to acquire guest resources

Certain memory resources associated with a guest are not necessarily
present in the guest P2M.

This patch adds the boilerplate for new memory op to allow such a resource
to be priv-mapped directly, by either a PV or HVM tools domain.

NOTE: Whilst the new op is not intrinsically specific to the x86 architecture,
      I have no means to test it on an ARM platform and so cannot verify
      that it functions correctly.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>