public / x86: introduce __HYPERCALL_dm_op...

public / x86: introduce __HYPERCALL_dm_op...

...as a set of hypercalls to be used by a device model.

As stated in the new docs/designs/dm_op.markdown:

"The aim of DMOP is to prevent a compromised device model from
compromising domains other then the one it is associated with. (And is
therefore likely already compromised)."

See that file for further information.

This patch simply adds the boilerplate for the hypercall.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Suggested-by: Ian Jackson <ian.jackson@citrix.com>
Suggested-by: Jennifer Herbert <jennifer.herbert@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>