s390x/virtio-hcall: Add range check for hypervisor call

The handler for diag 500 did not check whether the requested function
was in the supported range, so illegal values could crash QEMU in the
worst case.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
CC: qemu-stable@nongnu.org
