vlib: store buffer memory information in the buffer_main

Currently, buffer index is calculated as a offset to the physmem
region shifted by log2_cacheline size.

When DPDK is used we "hack" physmem data with information taken from
dpdk mempool. This makes physmem code not usable with DPDK.

This change makes buffer memory start and size independent of physmem
basically allowing physmem to be used when DPDK plugin is loaded.

Change-Id: Ieb399d398f147583b9baab467152a352d58c9c31
Signed-off-by: Damjan Marion <damarion@cisco.com>