xran's xran_bm_init() uses rte_pktmbuf_pool_create() to allocate DPDK
buffers. For a 1x1 configuration, the number of buffers might be too
small, and rte_pktmbuf_pool_create() might fail. This commit ensures
that we increase the number of buffers to have enough even in a 1x1
configuration. This works by chosing the next power of two, which is
recommended as per DPDK documentation.

Closes-Bug: #750