memory: add address_space_translate

Using phys_page_find to translate an AddressSpace to a MemoryRegionSection
is unwieldy.  It requires to pass the page index rather than the address,
and later memory_region_section_addr has to be called.  Replace
memory_region_section_addr with a function that does all of it: call
phys_page_find, compute the offset within the region, and check how
big the current mapping is.  This way, a large flat region can be written
with a single lookup rather than a page at a time.

address_space_translate will also provide a single point where IOMMU
forwarding is implemented.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
