intel-iommu: add context-cache to cache context-entry

Add context-cache to cache context-entry encountered on a page-walk. Each
VTDAddressSpace has a member of VTDContextCacheEntry which represents an entry
in the context-cache. Since devices with different bus_num and devfn have their
respective VTDAddressSpace, this will be a good way to reference the cached
entries.
Each VTDContextCacheEntry will have a context_cache_gen and the cached entry
is valid only when context_cache_gen equals IntelIOMMUState.context_cache_gen.

Signed-off-by: Le Tan <tamlokveer@gmail.com>
intel-iommu: add IOTLB using hash table

Add IOTLB to cache information about the translation of input-addresses. IOTLB
use a GHashTable as cache. The key of the hash table is the logical-OR of gfn
and source id after left-shifting.

Signed-off-by: Le Tan <tamlokveer@gmail.com>