xen: deal with pmd being allocated/freed

Deal properly with pmd-level pages being allocated and freed
dynamically.  We can handle them more or less the same as pte pages.

Also, deal with early_ioremap pagetable manipulations.

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