honor ACPI v4 FADT flags

honor ACPI v4 FADT flags

- force use of physical APIC mode if indicated so (as we don't support
  xAPIC cluster mode, the respective flag is taken to force physical
  mode too)
- don't use MSI if indicated so (implies no IOMMU)

Both can be overridden on the command line, for the MSI case this at
once adds a new command line option allowing to turn off PCI MSI (IOMMU
and HPET are unaffected by this).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>