mempool: rework support of Xen dom0

mempool: rework support of Xen dom0

Avoid to have a specific file for that, and remove #ifdefs.
Now that we have introduced a function to populate a mempool
with a virtual area, the support of xen dom0 is much easier.

The only thing we need to do is to convert the guest physical
address into the machine physical address using rte_mem_phy2mch().
This function does nothing when not running xen.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>