Add address info to sound, video and watchdog devices

Add address info to sound, video and watchdog devices

Add the virDomainDeviceAddress information to the sound, video
and watchdog devices. This means all of them gain the new XML
element

  <address .... />

This brings them upto par with disk/net/hostdev devices which
already have address info

* src/conf/domain_conf.h: Add virDomainDeviceAddress to sound,
  video & watchdog device struts.
* src/conf/domain_conf.c: Hook up parsing/formatting for
  virDomainDeviceAddress in sound, video & watchdog devices
* docs/schemas/domain.rng: Associate device address info
  with sound, video & watchdog