Fetch the root region from the sPAPRTCETable, and use it to build
an AddressSpace and DMAContext.

Now, everywhere we have a DMAContext we also have access to the
corresponding AddressSpace (either because we create it just before
the DMAContext, or because dma_context_memory's AddressSpace is
trivially address_space_memory).

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>