test: match qemu VIR_DOMAIN_DEF_FEATURE* usage
    
    Match the XML feature usage of the qemu driver, so the test driver
    doesn't reject things like <os firmware='efi'/>.
    
    Particularly VIR_DOMAIN_DEF_FEATURE_NET_MODEL_STRING is needed to
    prevent regressions for test suite users with net model strings that
    aren't in the virDomainNetModel enum yet
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>