Push legacy properties into a "legacy-..." namespace, and make them
available with correct types too.

For now, all properties come in both variants.  This need not be the
case for string properties.  We will revisit this after -device is
changed to actually use the legacy properties.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>