vhost: fix possible denial of service on SET_VRING_NUM

vhost: fix possible denial of service on SET_VRING_NUM

vhost_user_set_vring_num() performs multiple allocations
without checking whether data were previously allocated.

It may cause a denial of service because of the memory leaks
that happen if a malicious vhost-user master keeps sending
VHOST_USER_SET_VRING_NUM request until the slave runs out
of memory.

This issue has been assigned CVE-2019-14818

Fixes: b0a985d1f340 ("vhost: add dequeue zero copy")

Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>