memory: allow memory_region_find() to run on non-root memory regions

memory_region_find() is similar to registering a MemoryListener and
checking for the MemoryRegionSections that come from a particular
region.  There is no reason for this to be limited to a root memory
region.

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