netdev-dpdk: Use different constant for ring size

DPDK rings must have a power-of-two size.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Documentation: DPDK IVSHMEM VM Communications

Adds documentation on how to run IVSHMEM communication through VM.

Signed-off-by: Mike A. Polehn <mike.a.polehn@intel.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>