examples/vhost: move to safe GPA translation API

This patch uses the new rte_vhost_va_from_guest_pa() API
to ensure the application doesn't perform out-of-bound
accesses either because of a malicious guest providing an
incorrect descriptor length, or because the buffer is
contiguous in guest physical address space but not in the
host process virtual address space.

This issue has been assigned CVE-2018-1059.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>