VMXNET3: initialize rx_ridx to eliminate compile warning

  Gcc report "hw/vmxnet3.c:972: error: ‘rx_ridx’ may be used
uninitialized in this function", so fix it.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Message-id: 1364264646-27542-1-git-send-email-xiawenc@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
