memory: add a readonly attribute to MemoryRegionSection

.readonly cannot be obtained from the MemoryRegion, since it is
inherited from aliases (so you can have a MemoryRegion mapped RW
at one address and RO at another).  Record it in a MemoryRegionSection
for listeners.

Signed-off-by: Avi Kivity <avi@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>