VT-d: add "iommu=workaround_bios_bug" option

Add this option to workaround BIOS bugs. Currently it ignores DRHD
if "all" devices under its scope are not pci discoverable. This
workarounds a BIOS bug in some platforms to make VT-d work. But note
that this option doesn't guarantee security, because it might ignore
DRHD.

So there are 3 options which handle BIOS bugs differently:
  iommu=1 (default): If detect non-existent device under a DRHD's
scope, or find incorrect RMRR setting (base_address > end_address),
disable VT-d completely in Xen with warning messages. This guarantees
security when VT-d enabled, or just disable VT-d to let Xen work
without VT-d.
  iommu=force: it enforces to enable VT-d in Xen. If VT-d cannot be
enabled, it will crashes Xen. This is mainly for users who must need
VT-d.
  iommu=workaround_bogus_bios: it workarounds some BIOS bugs to make
VT-d still work.  This might be insecure because there might be a
device not protected by any DRHD if the device is re-enabled by
malicious s/w.  This is for users who want to use VT-d regardless of
security.

Signed-off-by: Weidong Han <weidong.han@intel.com>

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