This extension is described in

http://www.redhat.com/archives/libvir-list/2010-March/msg00304.html

Currently all attributes are optional, except name.

* src/conf/domain_conf.h: add data definition for virDomainTimerDef
  and add a list of them to virDomainClockDef
* src/conf/domain_conf.c: XML parser and formatter for a timer inside a clock
* src/libvirt_private.syms: add new Timer enum helper functions to symbols