net/virtio-user: fix devargs parsing
net/virtio-user: fix devargs parsing

strtoull returns 0 if it fails to parse input string. It's ignored
in get_integer_arg.

This patch handles error cases for strtoull function.

Fixes: ce2eabdd43ec ("net/virtio-user: add virtual device")
Cc: stable@dpdk.org

Signed-off-by: Ivan Dyukov <7201bbc590289db924bd55e2c7d4827e1373599f@samsung.com>
Reviewed-by: Maxime Coquelin <9a2667dee3b90866bbc9c2696cff2b7d5616c95a@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk