timers are sub-elements of clocks. A clock can have zero or more
instances of timer. Within the timer, only the name attribute is
required; all other attributes are optional.
A simpler representation of a timer element is:
<timer name='platform|pit|rtc|hpet|tsc'
wallclock='host|guest'
tickpolicy='delay|catchup|merge|discard'
frequency='123'
mode='auto|native|emulate|paravirt'
present='yes|no'/>
frequency is a ulong. All other attributes are simple enums.
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