spapr/rtas: Print message from "ibm,os-term"

The "ibm,os-term" RTAS call has a single parameter which is a pointer to
a message from the guest kernel about the termination cause; this prints
it.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20200203032044.118585-1-aik@ozlabs.ru>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr_rtas.c

@@ -345,6 +345,13 @@ static void rtas_ibm_os_term(PowerPCCPU *cpu,
                             target_ulong args,
                             uint32_t nret, target_ulong rets)
 {
+    target_ulong msgaddr = rtas_ld(args, 0);
+    char msg[512];
+
+    cpu_physical_memory_read(msgaddr, msg, sizeof(msg) - 1);
+    msg[sizeof(msg) - 1] = 0;
+
+    error_report("OS terminated: %s", msg);
     qemu_system_guest_panicked(NULL);
 
     rtas_st(rets, 0, RTAS_OUT_SUCCESS);