vhost: fix external backends readiness

vhost: fix external backends readiness

Commit d0fcc38f5fa4 ("vhost: improve device readiness notifications")
makes the assumption that every Virtio devices are considered
ready for preocessing as soon as first queue pair is configured
and enabled.

While this is true for Virtio-net, it isn't for Virtio-scsi
and Virtio-blk.

This patch fixes this by only making this assumption for
the builtin Virtio-net backend, and restores back to previous
behaviour for other backends.

Fixes: d0fcc38f5fa4 ("vhost: improve device readiness notifications")

Reported-by: Changpeng Liu <changpeng.liu@intel.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>