x86/xen: Add pvh specific rsdp address retrieval function

Add pvh_get_root_pointer() for Xen PVH guests to communicate the
address of the RSDP table given to the kernel via Xen start info.

This makes the kernel boot again in PVH mode after on recent Xen the
RSDP was moved to higher addresses. So up to that change it was pure
luck that the legacy method to locate the RSDP was working when
running as PVH mode.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: boris.ostrovsky@oracle.com
Cc: lenb@kernel.org
Cc: linux-acpi@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20180219100906.14265-4-jgross@suse.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>