core: map the zero page to the full hypervisor memory region

core: map the zero page to the full hypervisor memory region

During initialization, in init_early, the hypervisor maps the
memory used by the hypervisor with empty pages for the root cell.
However, if the root cell tries to access the region used by the
hypervisor, this is only safe if both sides agree on PAGE_SIZE.
It is a long shot to try to guess the granularity used by the
root cell; the safest bet is to map the full range that has been
allocated for the hypervisor to use.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>