docs: flesh out xl.cfg documentation, correct typos, reorganize

docs: flesh out xl.cfg documentation, correct typos, reorganize

Some highlights:
 * Correct some markup errors:
       Around line 663:
           '=item' outside of any '=over'
       Around line 671:
           You forgot a '=back' before '=head3'
 * Add documentation for msitranslate, power_mgnt, acpi_s3, aspi_s4,
   gfx_passthru, nomigrate, etc.
 * Reorganize items in "unclassified" sections like cpuid,
   gfx_passthru to where they belong
 * Correct link L<> references so they can be resolved within the
   document
 * Remove placeholders for deprecated options device_model and vif2
 * Remove placeholder for "sched" and "node", as these are options for
   cpupool configuration. Perhaps cpupool configuration deserves
   a section in this document.
 * Rename "global" options to "general"
 * Add section headers to group general VM options.

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