xen: efficiently support a holey p2m table

When using sparsemem and memory hotplug, the kernel's pseudo-physical
address space can be discontigious.  Previously this was dealt with by
having the upper parts of the radix tree stubbed off.  Unfortunately,
this is incompatible with save/restore, which requires a complete p2m
table.

The solution is to have a special distinguished all-invalid p2m leaf
page, which we can point all the hole areas at.  This allows the tools
to see a complete p2m table, but it only costs a page for all memory
holes.

It also simplifies the code since it removes a few special cases.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>