"-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: NChristian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>