In QEMU PPC64 we have a network device called "spapr-vlan". We can specify
this using the existing syntax for network devices, however libvirt
currently rejects "spapr-vlan" in virDomainNetDefParseXML() because of
the "-". Fix the code to accept "-".

* src/conf/domain_conf.c (virDomainNetDefParseXML): Allow '-' in
model name, and be more efficient.
* docs/schemas/domaincommon.rng: Limit valid model names to match code.
Based on a patch by Michael Ellerman.