quickstart: inform ignorable VPP warnings and errors

When VPP boots up, it displays warnings about unsupported PCI devices
and errors about plugins which are not loaded by default. There might
be other errors which can be ignored safely for the use cases
in quick start guide.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>

Change-Id: Idd3d302641bcae14a5b81268706c45a38b9e6414