core: Introduce cell shutdown control interface

core: Introduce cell shutdown control interface

Using the messaging feature of the comm region, this introduces an
interface for cell to control their shutdown. The hypervisor sends a
shutdown request to a cell before destroying it. The cell can either
reject or accept the request in its reply message.

Cells can be excluded from this procedure and will continue to be
destructed immediately by setting the flag JAILHOUSE_CELL_UNMANAGED_EXIT
in their configuration file. For now we mark both demo cells as
unmanaged.

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