virDomainSaveConfig calls virDomainDefFormat which was setting the caps
to NULL, thus keeping the old behaviour (i.e. not looking at
netprefix). This patch adds the virCapsPtr to the function and allows
the configuration to be saved and skipping interface names that were
registered with virCapabilitiesSetNetPrefix().
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>