devargs: allow to provide arguments per pci device

devargs: allow to provide arguments per pci device

Some PCI drivers may require some specific initialization arguments at
start-up.

Even if unused today, adding this feature seems coherent with virtual
devices in order to provide a full-featured rte_devargs framework. In
the future, it could be added in pmd_ixgbe or pmd_igb for instance to
enable debug of drivers or setting a specific operating mode at
start-up.

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