schema: Fix guest timer specification schema according to the docs

schema: Fix guest timer specification schema according to the docs

According to the documentation describing various tunables for domain
timers not all the fields are supported by all the driver types. Express
these in the RNG:

- rtc, platform: Only these support the "track" attribute.
- tsc: only one to support "frequency" and "mode" attributes
- hpet, pit: tickpolicy/catchup attribute/element
- kvmclock: no extra attributes are supported

Additionally the attributes of the <catchup> element for
tickpolicy='catchup' are optional according to the parsing code. Express
this in the XML and fix a spurious space added while formatting the
<catchup> element and add tests for it.