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 <firstname.lastname@example.org>
Signed-off-by: Cole Robinson <email@example.com>