DPDK: fix crash at dpdk_virtio_from_vm_rx
DPDK: fix crash at dpdk_virtio_from_vm_rx

The crash was due to the empty (zeroed out) descriptors we read from
the virtio queue.

+ Return empty and invalid descriptors to the driver (guest).

Change-Id: I2e5a8e181b4a8c4e165b8474b0bdb75459884c2d
Closes-bug: #1509171

codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter