core: Make sure Linux cell gets domain ID 0

cell_init will invoke get_free_cell_id also for the Linux cell, thus
comparing ID 0 against the current value of linux_cell.id. Set it to an
invalid ID before initializing that cell so that ID 0 is used for the
first cell.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
x86: Wait for VT-d shutdown

If disabling the DMAR unit actually took effect is visible in the status
register. We need to poll for the TE bit before resuming to ensure that
no memory-resident tables are still used by the hardware.

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