dpdk: terminate device devargs string

dpdk: terminate device devargs string

Type: fix

When a device is whitelisted with devargs arguments specified, the
string that is generated and added to conf->eal_init_args is not
explicitly terminated with 0. If the formatted string takes up all
of the memory allocated to the vector which stores it and it is
used later as a string in a format() or printf() call, any nonzero
characters stored in memory at the address immediately following the
memory allocated for the vector will be erroneously appended to the
string.

Terminate the string with 0 to ensure that this does not happen.

Change-Id: I20a78d994daad93bf5aecab5c03d705022e882ec
Signed-off-by: Matthew Smith <mgsmith@netgate.com>