x86: adjust/fix LDT handling for Xen

Based on patch from Jan Beulich <jbeulich@novell.com>.

Don't rely on kmalloc(PAGE_SIZE) returning PAGE_SIZE aligned memory
(Xen requires GDT *and* LDT to be page-aligned). Using the page
allocator interface also removes the (albeit small) slab allocator
overhead. The same change being done for 64-bits for consistency.

Further, the Xen hypercall interface expects the LDT address to be
virtual, not machine.

[ Adjusted to unified ldt.c - Jeremy ]

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>