qom: allow creating an alias of a child<> property

qom: allow creating an alias of a child<> property

Child properties must be unique.  Fix this problem by
turning their aliases into links.

The resolve function that forwards to the target property
does not have any knowledge of the target property's type,
so it works fine.

Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>