examples/vhost_scsi: move to safe GPA translation API

examples/vhost_scsi: move to safe GPA translation API

This patch uses the new rte_vhost_va_from_guest_pa() API
to ensure all the descriptor buffer is mapped contiguously
in the application virtual address space.

As the application did not checked return of previous API,
this patch just print an error if the buffer address isn't in
the vhost memory regions or if it is scattered. Ideally, it
should handle scattered buffers gracefully.

This issue has been assigned CVE-2018-1059.

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