virtio-net: expose virtio_net_supported_guest_offloads()

virtio-net: expose virtio_net_supported_guest_offloads()

To support restoring offloads state in vdpa, it is necessary to
expose the function virtio_net_supported_guest_offloads().

According to VirtIO standard, "Upon feature negotiation
corresponding offload gets enabled to preserve backward compatibility.".
Therefore, QEMU uses this function to get the device supported offloads.
This allows QEMU to know the device's defaults and skip the control
message sending if these defaults align with the driver's configuration.

Note that the device's defaults can mismatch the driver's configuration
only at live migration.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <43679506f3f039a7aa2bdd5b49785107b5dfd7d4.1685704856.git.yin31149@gmail.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Tested-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>