memory: split address_space_read and address_space_write

Rather than dispatching on is_write for every iteration, make
address_space_rw call one of the two functions.  The amount of
duplicate logic is pretty small, and memory_access_is_direct can
be tweaked so that it inlines better in the callers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>