xen: fix retrieval of physical address

xen: fix retrieval of physical address

When using Xen Dom0, it looks that /proc/self/pagemap returns 0.
This breaks the creation of mbufs pool.

We can workaround this in rte_mem_virt2phy() by browsing the dpdk memory
segments. This only works for dpdk memory, but it's enough to fix the
mempool creation.

Fixes: c042ba20674a ("mempool: rework support of Xen dom0")
Fixes: 3097de6e6bfb ("mem: get physical address of any pointer")

Reported-by: Huilong Xu <huilongx.xu@intel.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>