hw/s390x/s390-virtio-ccw: Improve the machine description string

hw/s390x/s390-virtio-ccw: Improve the machine description string

The machine name already contains the words "ccw" and "virtio", so
using "VirtIO-ccw" in the description likely does not really help
the average user to get an idea what this machine type is about.
Thus let's switch to "Virtual s390x machine" now, since "virtual
machine" should be a familiar term, and "s390x" signals that this
is about 64-bit guests (unlike S390 which could mean that it is
31-bit only).
Also expand "v" to "version", since this makes it easier to use
this macro also with non-numeric machine names in downstream.

Message-Id: <20220506065026.513590-1-thuth@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>