libxc/x86: fix page table creation for huge guests

libxc/x86: fix page table creation for huge guests

The switch-over logic from one page directory to the next was wrong;
it needs to be deferred until we actually reach the last page within
a given region, instead of being done when the last entry of a page
directory gets started with.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>