core, driver: Pass rounded-up core size in hypervisor header

core, driver: Pass rounded-up core size in hypervisor header

Hypervisor and root kernel may have different ideas about PAGE_SIZE.
This will cause wrong hypervisor core size calculations as seen on arm64
with 64K Linux PAGE_SIZE.

Avoid this trap by moving the round-up into the hypervisor code, passing
a ready-to-be-used size value in the header.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>