devargs: make devargs list private

devargs: make devargs list private

Initially, rte_devargs was meant to be populated once and sometimes
accessed, then never emptied.

With the new hotplug functionality having better standing, new usage
appeared with repeated addition of devices and their subsequent removal.

Exposing devargs_list pushed bus drivers and libraries to be careless
and inconsistent in their memory management. Making it private will
allow to rationalize this part of the EAL and ensure that fewer memory
leaks occur during operations.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>