s390-virtio: Check for NULL device in reset hypercall

s390_virtio_bus_find_mem() may return a NULL VirtIOS390Device.
If called with, e.g., args[0] == 0, this leads to a segfault.
Fix this by adding error handling as done for other hypercalls.

Present since baf0b55a9e57b909b1f8b0f732c0b10242867418 (Implement
virtio reset).

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
