iotests: make 235 work on s390 (and others)

iotests: make 235 work on s390 (and others)

"-machine pc" will not work all architectures. Lets fall back to the
default machine by not specifying it.

In addition we also need to specify -no-shutdown on s390 as qemu will
exit otherwise.

Cc: qemu-stable@nongnu.org
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 2c26e648e4350079b0c86a6627b2d3566c3709c0)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>