virterror: Misleading error message when name is missing

virterror: Misleading error message when name is missing

[forwarding this here from RH bug #796732]

When creating a network (virsh net-create) with an erroneous XML
containing an empty <name> element, the error message is misleading:

error: Failed to create network from foo.xml
error: missing domain name information

It took me a bit of time to figure out that it was the *network* name
that was missing (I generate this xml and didn't look at it, first).

I realized that the same message is used for missing name when creating
a domain, network, or device node.