virtio_net: disable VIRTIO_NET_F_STANDBY if VIRTIO_NET_F_MAC is not set

failover relies on the MAC address to pair the primary and the standby
devices:

  "[...] the hypervisor needs to enable VIRTIO_NET_F_STANDBY
   feature on the virtio-net interface and assign the same MAC address
   to both virtio-net and VF interfaces."

  Documentation/networking/net_failover.rst

This patch disables the STANDBY feature if the MAC address is not
provided by the hypervisor.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>