[libvirt] When bridge doesn't exist

["Anton Protopopov" <aspsk2 gmail com>]

Assume that we want to add a bridge to LAN to a guest.
We saying to libvirt:

...
<interface type="bridge">
    <source bridge="BRIDGE">
</interface>
...

What is the right behaviour of libvirt in case bridge BRIDGE
doesn't exist---return an error or try to create that bridge?
If it is the former, then who must create that bridge?