intel-iommu: Avoid global flushes with caching mode.

While it may be efficient on real hardware, emulation of global
invalidations is very expensive as all shadow entries must be examined.
This patch changes the behaviour when caching mode is enabled (which is
the case when IOMMU emulation takes place). In this case, page specific
invalidation is used instead.

Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>