bus: fix device iterator match from arguments

bus: fix device iterator match from arguments

Device iterator RTE_DEV_FOREACH() failed to return devices from
classifier like "class=vdpa", because matching name from empty kvargs
returns no result. If device name not specified in kvargs, the function
should iterate all devices.

This patch allows empty devargs or devargs without name specified.

Fixes: 6aebb942907d ("kvargs: add function to get from key and value")

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Xueming Li <xuemingl@nvidia.com>