tboot, xen: Update for Trusted Boot v20071128.

tboot, xen: Update for Trusted Boot v20071128.

This patch updates the Xen to work with the latest version (20071128)
of Trusted Boot (tboot).  This version of tboot now resides at 16MB
(instead of the previous <1MB), in addition to several other
enhancements.  By residing at 16MB, this version of tboot will be
protected from access by dom0.

This patch allows Xen to correctly map the tboot shutdown code that it
must trampoline into for a clean shutdown (without this patch Xen will
fault on shutdown).  This patch will also work with the previous
version of tboot.

Signed-off-by: Joseph Cihula <joseph.cihula@intel.com>