Allow a timezone to be specified instead of sync to host timezone

Allow a timezone to be specified instead of sync to host timezone

This extends the XML to allow for

  <clock offset='timezone' timezone='Europe/Paris'/>

This is useful if the admin has not configured any timezone on the
host OS, but still wants to synchronize a guest to a specific one.

* src/conf/domain_conf.h, src/conf/domain_conf.c: Support extra
  'timezone' attribute on clock configuration
* docs/schemas/domain.rng: Add 'timezone' attribute
* src/xen/xend_internal.c, src/xen/xm_internal.c: Reject configs
  with a configurable timezone