vhost-user: clarify start and enable

vhost-user: clarify start and enable

It seems that we currently have some duplication between
started and enabled states.

The actual reason is that enable is not documented correctly:
what it does is connecting ring to the backend.

This is important for MQ, because a Linux guest expects TX
packets to be completed even if it disables some queues
temporarily.

Cc: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: Victor Kaplansky <victork@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>