virtio-{blk,balloon,net,serial} are aliases for their actual,
architecture-dependent implementations (*-ccw on s390x, *-pci on other
architectures supporting virtio). This makes it a lot easier to craft
qemu invocations that work on all supported architectures. Complete
the set to cover all existing non-abstract virtio device classes.

For virtio-balloon, only the CCW implementation was missing.
Signed-off-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1455831854-49013-4-git-send-email-silbe@linux.vnet.ibm.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>