docs/html/: Annotations for two hypercalls

docs/html/: Annotations for two hypercalls

Add annotations for a couple of the hypercalls:
 HYPERVISOR_set_trap_table
 HYPERVISOR_mmu_update

We do this by
 * annotating the list of #defines for hypercall numbers
 * annotating the list of error values
 * providing a function prototype for the systematically-named functions
The header generator does the rest.

This exercise revealed a couple of infelicities:
 * In the actual source code, do_mmu_update is defined to return an int
   and do_set_trap_table a long.  However both functions return either
   -Efoo (on error) or 0 for success.
 * The error numbers are defined only in the private header file
   xen/include/xen/errno.h and then only with names which will
   typically clash with other projects.  It would be nice to include a
   public version of this header which defines XEN_E*.  But for now
   we run xen-headers on errno.h too.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>