[S390] sparse: fix sparse symbol shadow warning

to_kvmdev and dev_to_virtio both use container_of. Avoid to nest the
two macros to quiet the following sparse warning:

drivers/s390/kvm/kvm_virtio.c:337:20: warning: symbol '__mptr' shadows an earlier one
drivers/s390/kvm/kvm_virtio.c:337:20: originally declared here

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>