vtd: Only enable some VT-d features if all VT-d engines support them.

vtd: Only enable some VT-d features if all VT-d engines support them.

By default, we enable snoop control, queued invalidation and interrupt
remapping if all VT-d engines support them, and for DMA passthrough we
don't enable it by default.

A user can use 'iommu=passthrough' to enable DMA passthrough (only
for Dom0). A user can use 'iommu=no-snoop,no-qinval,no-intremap' to
disable the 3 features.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>=