vdpa: Read device configuration only if FEATURES_OK

Avoid reading device configuration during feature negotiation. Read
device status and verify that VIRTIO_CONFIG_S_FEATURES_OK is set.

Protect the entire operation, including configuration read with cf_mutex
to ensure integrity of the results.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-5-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>