resubmit of fixes

modified:   examples/udp_echo_server/udp_echo_server.c

    Fixed bug, the buffer was not echo'ed as supposed too. An empty buffer was send, instead of the received message

modified:   src/libdpdkif/dpdkif_user.c
    Modifications made to make dpdkif_user work with the vmxnet3 driver:
    -No support for TX Multisegments
    -No support for TX Offload
    -The vmxnet3 needs at least 512 TX descriptors and recommends at least 256 RX descriptor
    -The amount of mbufs in the defined rx&tx pools should contain equal amount of rx or tx descriptors + a certain amount extra to cope with packet