vhost: fix max queues

vhost: fix max queues

0x8000 is the max virito-net queue pairs the virtio 1.0 spec claims to
support. While for vhost-user, it's a different story: the max vring
index could be passed by the vhost-user spec is 0xff, masked by the
VHOST_USER_VRING_IDX_MASK.

That said, the max queue pairs could vhost-user could supported is 0x80.
If user are asking more, I think the vhost-user need be extended.

Fixes: b09b198bfb5c ("vhost-user: announce queue number in message")
Cc: stable@dpdk.org
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>