exec: change iotlb APIs to take AddressSpaceDispatch

This makes it possible to start following RCU rules, which require
not dereferencing as->dispatch more than once.  It is not covering
the whole of TCG, since the TLB data structures are not RCU-friendly,
but it is enough for exec.c.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
fixup changes to docs/rcu.txt from Paulo Bonzini commit: 69d44653