vdev: new registration API

vdev: new registration API

Instead of having a list of virtual device drivers in EAL code, add an
API to register drivers. Thanks to this new registration method, we can
remove the references to pmd_ring, pmd_pcap and pmd_xenvirt in EAL code.
This also enables the ability to register a virtual device driver as
a shared library.

The registration is done in an init function flaged with
__attribute__((constructor)). The new convention is to name this
function rte_pmd_xyz_init(). The per-device init function is renamed
rte_pmd_xyz_devinit().

By the way the internal PMDs are now also .so/standalone ready. Let's do
it later on. It will be required to ease maintenance.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>