intel-iommu: add reserved field check for Invalidation Wait Descriptor

Add reserved field check for Invalidation Wait Descriptor in
vtd_process_wait_desc(). If they are non-zero, invalidation queue error
happens.

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