vhost: get guest/host physical address mappings

vhost: get guest/host physical address mappings

So that we can convert a guest physical address to host physical
address, which will be used in later Tx zero copy implementation.

MAP_POPULATE is set while mmaping guest memory regions, to make
sure the page tables are setup and then rte_mem_virt2phy() could
yield proper physical address.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>