vdev: allow external registration of virtual device drivers

vdev: allow external registration of virtual device drivers

The registration of an external vdev driver (a .so library) is done in a
function that has the ((constructor)) attribute. This function is called
when dlopen(driver.so) is invoked.

As a result, we need to do the dlopen() before calling
rte_eal_vdev_init() that calls the initialization functions of all
registered drivers.

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