x86: Add fault reporting in VT-d

The reporting facilitates configuration of PCI-devices. If there's
an error then corresponding message is shown in a console.
The reporting is implemented as delivery of NMI-interrupts via MSI to
one of the Linux cell's cores.

Signed-off-by: Ivan Kolchin <ivan.kolchin@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Adjust error codes of cell creation

Consistently report -EBUSY if a new cell configuration contains CPUs
that are used for other cells already.

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