s390x: Register TYPE_S390_CCW_MACHINE properties as class properties

s390x: Register TYPE_S390_CCW_MACHINE properties as class properties

Currently, when running 'qemu-system-s390x -M s390-ccw-virtio,help'
the s390x-specific properties are not listed anymore. This happens
because since commit d8fb7d0969 ("vl: switch -M parsing to keyval")
the properties have to be defined at the class level and not at the
instance level anymore. Fix it on s390x now, too, by moving the
registration of the properties to the class level"

Fixes: d8fb7d0969 ("vl: switch -M parsing to keyval")
Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Message-Id: <20221103170150.20789-2-pmorel@linux.ibm.com>
[thuth: Add patch description]
Signed-off-by: Thomas Huth <thuth@redhat.com>