address_space_rw is calling address_space_to_flatview but it can
be called outside the RCU lock.  To fix it, transform flatview_rw
into address_space_rw, since flatview_rw is otherwise unused.

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>