S390: fix kernel_commandline handling

The current handling of kernel parameters is broken. The pointer
is always valid, even if no -kernel or -append is specified.
We must check if the kernel rom address is valid instead,
otherwise qemu might segfault.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
