net/virtio-user: fix overflow

net/virtio-user: fix overflow

virtio-user limits the qeueue number to 8 but provides no limit
check against the queue number input from user. If a bigger queue
number (> 8) is given, there is an overflow issue. Doing a sanity
check could avoid it.

Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")
Cc: stable@dpdk.org
Signed-off-by: Wenfeng Liu <liuwf@arraynetworks.com.cn>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>