libxl: fix rtc_timeoffset setting

libxl: fix rtc_timeoffset setting

libxl__domain_build_info_setdefault may be called several times,
so rtc_timeoffset can't be setted in it.

Move rtc_timeoffset setting logic to libxl__build_pre.

Reported-by: Teck Choon Giam <giamteckchoon@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Lin Ming <mlin@ss.pku.edu.cn>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
---
 tools/libxl/libxl_create.c |    9 ---------
 tools/libxl/libxl_dom.c    |   17 +++++++++++++++--
 2 files changed, 15 insertions(+), 11 deletions(-)