Documentation: Add hypervisor header field descriptions

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Documentation: Add references to memory-layout.txt

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Remove caller restriction from Hypervisor Get Info hypercall

There is no sensitive information to hide here. Rather, we want to be
able to use this hypercall also from non-root cells, e.g. to monitor the
system setup progress.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Provide number of registered cells via hypercall

Extend the Hypervisor Get Info hypercall to also provide the number of
registered cell (including the root cell).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
x86: Fix build on 32-bit userlands

If building for a 32-bit target userland, the toolchain defaults to
32-bit as well. While we gain -m64 automatically while building the
kernel module, we need to inject it explicitly for the hypervisor and
the inmates.

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