automation: qemu-smoke-arm64: Silence ifconfig error messages

automation: qemu-smoke-arm64: Silence ifconfig error messages

During the ping test, dom1 tries to assign an ip to eth0 in a loop.
Before setting up the network interface by dom0, this results in
printing the following error message several times:
(XEN) DOM1: ifconfig: SIOCSIFADDR: No such device

Silence this by redirecting stderr/stdout to /dev/null as we do not
care about the output and we should not pollute the log file.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Ayan Kumar Halder <ayankuma@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>