aml-build: don't modify child

aml-build: don't modify child

this code:
    aml_append(foo, bar);

might, non-intuitively, modify bar, which means that e.g. the following
might not DTRT:

    c = ....;
    aml_append(a, c);
    aml_append(b, c);

to fix, simply allocate an intermediate array,
and always modify that.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>