hw/s390x/ipl: Bail out if the network bootloader can not be found

hw/s390x/ipl: Bail out if the network bootloader can not be found

If QEMU fails to load 's390-netboot.img', the guest firmware currently
loops forever and just floods the console with "Network boot device
detected" messages. The code in ipl.c apparently already tried to stop
the VM with vm_stop() in this case, but this is in vain since the run
state is later reset due to a call to vm_start() from vl.c again.
To avoid the ugly firmware loop, let's simply exit QEMU directly instead
since it just does not make sense to continue if the required firmware
image can not be loaded. While we're at it, also add the file name of
the netboot binary to the error message, so that the user has a better
hint about what is missing.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1519725913-24852-1-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>