xl.cfg man page cleanup and fixes

xl.cfg man page cleanup and fixes

- fixed some minor numbering and syntax issues in the CPU allocation
  examples for the 'cpus' option
- semantic fixes to make explanations more clear throughout
- fixed all the typo's I could see
- general styling and makeup fixes to make everything look more consistent

Signed-off-by: Armando Vega <armando@greenhost.nl>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>