This patch fix three simple errors that are preventing libvhost-user
from building.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>

When configuring VQs, Qemu will tell the backend where to start
processing the avail ring with a SET_VRING_BASE message. However, there
is no explicit message to tell the backend where to start processing the
used ring.

This patch updates the local copy of a vq's used_idx upon seeing a
SET_VRING_ADDR message. The used ring may have changed if the driver has
been reloaded, for example, or between SeaBIOS using the device and then
the kernel loading the relevant virtio module.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>

The shadow copy of the last_avail_idx must also be updated upon
receiving a SET_VRING_BASE message.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>

Currently, VQs are only marked as started or stopped when
queue_set_started() is configured during vu_init(). This patch mark VQs
as started/stopped regardless. This field might be useful for other
changes in the near future.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>

Currently, VQs are started as soon as a SET_VRING_KICK is received. That
is too early in the VQ setup process, as the backend might not yet have
a callfd to notify in case it received a kick and fully processed the
request/command. This patch only starts a VQ when a SET_VRING_CALL is
received.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>