doc: announce API changes for vhost

doc: announce API changes for vhost

I made a vhost ABI/API refactoring at v16.04, meant to avoid such issue
forever. Well, apparently, I lied.

People are looking for more vhost-user options now days, other than
vhost-user net only. For example, SPDK (Storage Performance Development
Kit) are looking for chance of vhost-user SCSI and vhost-user block.

Apparently, they also need a vhost-user backend, while DPDK already
has a (mature enough) backend, they don't want to implement it again
from scratch. They want to leverage the one DPDK provides.

However, the last refactoring hasn't done that right, at least it's
not friendly for extending vhost-user to add more devices support.
For example, different virtio devices has its own feature set, while
APIs like rte_vhost_feature_disable(feature_mask) have no option to
tell the device type. Thus, a more proper API should look like:

    rte_vhost_feature_disable(device_type, feature_mask);

Besides that, few public files and structures should be renamed, to
not let it bind to virtio-net. Specifically, they are:

- virtio_net_device_ops --> vhost_device_ops
- rte_virtio_net.h      --> rte_vhost.h

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>