Currently we use a global radix tree to dispatch memory access.  This only
works with a single address space; to support multiple address spaces we
make the radix tree a member of AddressSpace (via an intermediate structure
AddressSpaceDispatch to avoid exposing too many internals).

A side effect is that address_space_io also gains a dispatch table.  When
we remove all the pre-memory-API I/O registrations, we can use that for
dispatching I/O and get rid of the original I/O dispatch.

Signed-off-by: Avi Kivity <avi@redhat.com>