virtio-net: fix offload ctrl endian

Spec said offloads should be le64, so use virtio_ldq_p() to guarantee
valid endian.

Fixes: 644c9858 ("virtio-net: dynamic network offloads configuration")
Cc: qemu-stable@nongnu.org
Cc: Dmitry Fleytman <dfleytma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>