qom: push permission checks up into qdev_property_add_legacy

qdev_property_get and qdev_property_set can generate permission
denied errors themselves.  Do not duplicate this functionality in
qdev_get/set_legacy_property, and clean up excessive indentation.

Reviewed-by: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
