x86/boot: print CPU and APIC ID in bring up failure

Print the CPU and APIC ID that fails to respond to the init sequence, or
that didn't manage to reach the "callin" state.  Expand a bit the printed
error messages.  Otherwise the "Not responding." message is not easy to
understand by users.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/smpboot.c

@@ -621,10 +621,12 @@ static int do_boot_cpu(int apicid, int cpu)
             smp_mb();
             if ( bootsym(trampoline_cpu_started) == 0xA5 )
                 /* trampoline started but...? */
-                printk("Stuck ??\n");
+                printk("APIC ID %#x (CPU%u) didn't finish start sequence\n",
+                       apicid, cpu);
             else
                 /* trampoline code not run */
-                printk("Not responding.\n");
+                printk("APIC ID %#x (CPU%u) didn't respond to SIPI\n",
+                       apicid, cpu);
         }
     }