tools/virtio/ringtest: fix run-on-all.sh to work without /dev/cpu

/dev/cpu is only available on x86 with certain modules (e.g. msr) enabled.
Using lscpu to get processors count is more portable.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>