libxc: rework of domain builder's page table handler

libxc: rework of domain builder's page table handler

In order to prepare a p2m list outside of the initial kernel mapping
do a rework of the domain builder's page table handler. The goal is
to be able to use common helpers for page table allocation and setup
for initial kernel page tables and page tables mapping the p2m list.
This is achieved by supporting multiple mapping areas. The mapped
virtual addresses of the single areas must not overlap, while the
page tables of a new area added might already be partially present.
Especially the top level page table is existing only once, of course.

Currently restrict the number of mappings to 1 because the only mapping
now is the initial mapping created by toolstack. There should not be
behaviour change and guest visible change introduced.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>