x86: init_hypercall_page() cleanup

x86: init_hypercall_page() cleanup

The various pieces of the hypercall page infrastructure have grown
organically over time and ended up in a bit of a mess.

 * Rename all functions to be of the form *_init_hypercall_page().  This
   makes them somewhat shorter, and means they can actually be grepped
   for in one go.
 * Move init_hypercall_page() to domain.c.  The 64-bit traps.c isn't a
   terribly appropriate place for it to live.
 * Drop an obsolete comment from hvm_init_hypercall_page() and drop the
   domain parameter from hvm_funcs.init_hypercall_page() as it isn't
   necessary.
 * Rearrange the logic in the each function to avoid needing extra local
   variables, and to write the page in one single pass.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>