Change the internal domain conf representation of localtime/utc

Change the internal domain conf representation of localtime/utc

The XML will soon be extended to allow more than just a simple
localtime/utc boolean flag. This change replaces the plain
'int localtime' with a separate struct to prepare for future
extension

* src/conf/domain_conf.c, src/conf/domain_conf.h: Add a new
  virDomainClockDef structure
* src/libvirt_private.syms: Export virDomainClockOffsetTypeToString
  and virDomainClockOffsetTypeFromString
* src/qemu/qemu_conf.c, src/vbox/vbox_tmpl.c, src/xen/xend_internal.c,
  src/xen/xm_internal.c: Updated to use new structure for localtime