vhost: add missing experimental flag

vhost: add missing experimental flag

This function is listed under EXPERIMENTAL in the
rte_vhost_version.map, so it needs to be marked
with __rte_experimental in the header file as well.

Found by check-experimental-syms.sh when trying to compile
DPDK with -finstrument-functions.  This script didn't
catch this in the normal case, since the function is
declared __rte_always_inline.

This also requires updating the vhost_scsi example to allow
use of this newly marked experimental API.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>