memory: seek FlatView sharing candidates among children subregions

A container can be used instead of an alias to allow switching between
multiple subregions.  In this case we cannot directly share the
subregions (since they only belong to a single parent), but if the
subregions are aliases we can in turn walk those.

This is not enough to remove all source of quadratic FlatView creation,
but it enables sharing of the PCI bus master FlatViews (and their
AddressSpaceDispatch structures) across all PCI devices.  For 112
virtio-net-pci devices, boot time is reduced from 25 to 10 seconds and
memory consumption from 1.4 to 1 G.

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

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