x86_64: don't allocate L1 per-domain page table pages in a single chunk

x86_64: don't allocate L1 per-domain page table pages in a single chunk

Instead, allocate them on demand, and adjust the consumer to no longer
assume the allocated space is contiguous.

This another prerequisite to extend to number of vCPU-s the hypervisor
can support per guest.

Signed-off-by: Jan Beulich <jbeulich@novell.com>