memory: fix refcount leak in memory_region_present

memory_region_present() leaks a reference to a MemoryRegion in the
case "mr == container".  While fixing it, avoid reference counting
altogether for memory_region_present(), by using RCU only.

The return value could in principle be already invalid immediately
after memory_region_present returns, but presumably the caller knows
that and it's using memory_region_present to probe for devices that
are unpluggable, or something like that.  The RCU critical section
is needed anyway, because it protects as->current_map.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>