vhost: remove a hack on queue allocation

vhost: remove a hack on queue allocation

We used to allocate queues based on the index from SET_VRING_CALL
request: if corresponding queue hasn't been allocated, allocate it.

Though it's pratically right (it's the first per-vring request we
will get from QEMU for vhost-user negotiation), but it's not technically
right: it's not documented in the vhost-user spec that it will always
be the first per-vring request. For example, SET_VRING_ADDR could also
be the first per-vring request.

Thus, we should not depend the SET_VRING_CALL on queue allocation.
Instead, we could catch all the per-vring messages at the entrance of
request handler, and allocate one if it hasn't been allocated before.

By that, we could remove a hack.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>