vhost: check ring descriptor address

vhost: check ring descriptor address

In current implementation vhost will crash with segmentation fault
if malicious or buggy virtio application breaks addresses of descriptors.

Before commit 0823c1cb0a73 ("vhost: workaround stale vring base")
this crash was reproducible even with normal DPDK application that tries
to change number of virtqueues dynamically inside VM.

Fix that by checking addresses of descriptors before using.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>