tools/docs: Remove PVRDTSCP support

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 c17b36d5dc792cfdf59b6de0213b168bec0af8e8
  c/s 04656384a1b9714e43db850c51431008e23450d8

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>