tools/docs: Remove PVRDTSCP support

PVRDTSCP is believed-unused, and its implementation has adverse consequences
on unrelated functionality in the hypervisor.  As a result, support is being
removed.

For more historical context, see
  c/s c17b36d5dc
  c/s 04656384a1

Modify libxl to provide a slightly more helpful error message if it encounters
PVRDTSCP being selected.  While adjusting TSC handling, make libxl check for
errors from the set_tsc hypercall.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/python/xen/lowlevel/xc/xc.c

@@ -2439,7 +2439,7 @@ static PyMethodDef pyxc_methods[] = {
       "Set a domain's TSC mode\n"
       " dom        [int]: Domain whose TSC mode is being set.\n"
       " tsc_mode   [int]: 0=default (monotonic, but native where possible)\n"
-      "                   1=always emulate 2=never emulate 3=pvrdtscp\n"
+      "                   1=always emulate 2=never emulate\n"
       "Returns: [int] 0 on success; -1 on error.\n" },
 
     { "domain_disable_migrate",