spapr: convert TCE API to use an opaque type

The TCE table is currently returned as a DMAContext, and non-type-safe
APIs are called later passing back the DMAContext.  Since we want to move
away from DMAContext, use an opaque type instead, and add an accessor
to retrieve the DMAContext from it.

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