Merge remote-tracking branch 'remotes/jasowang/tags/net-pull-request' into staging

# gpg: Signature made Mon 25 Nov 2019 15:30:56 GMT
# gpg:                using RSA key EF04965B398D6211
# gpg: Good signature from "Jason Wang (Jason Wang on RedHat) <jasowang@redhat.com>" [marginal]
# gpg: WARNING: This key is not certified with sufficiently trusted signatures!
# gpg:          It is not certain that the signature belongs to the owner.
# Primary key fingerprint: 215D 46F4 8246 689E C77F  3562 EF04 965B 398D 6211

* remotes/jasowang/tags/net-pull-request:
  net/virtio: return error when device_opts arg is NULL
  net/virtio: fix re-plugging of primary device
  net/virtio: return early when failover primary alread added
  net/virtio: fix dev_unplug_pending

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>