Move the creation of QmpInputVisitor and QmpOutputVisitor from qmp.c
to qom/object.c, since it's the only practical way to access object
properties.

Keep this isolated such that it's easy to remove.  At some point, we need
to remove all usage of QObject in the tree and replace it with GVariant.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>