x86: Factor out vmx_map_memory_region

We will reuse the function during cell destruction.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
x86: Factor out flush_linux_cpu_caches

Will be reused for arch_cell_destroy.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Implement cell destruction

When a cell is destroyed, we need to stop its CPUs and assigned all
its resources that Linux used to have before back to that cell. Like for
shutdown, access control is still missing.

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