The capabilities XML uses the x86 specific terms 'S3', 'S4'
and 'Hybrid-Syspend'. Switch it to use the same terminology
as the API constants and virsh options, eg 'suspend_mem'
'suspend_disk' and 'suspend_hybrid'

* docs/formatcaps.html.in, docs/schemas/capability.rng,
  src/conf/capabilities.c: Rename suspend constants