dump-guest-memory.py: fix "You can't do that without a process to debug"

If the script is run with a core (no running process), it produces an
error:

(gdb)  dump-guest-memory /tmp/vmcore X86_64
guest RAM blocks:
target_start     target_end       host_addr        message count
---------------- ---------------- ---------------- ------- -----
0000000000000000 00000000000a0000 00007f7935800000 added       1
00000000000a0000 00000000000b0000 00007f7934200000 added       2
00000000000c0000 00000000000ca000 00007f79358c0000 added       3
00000000000ca000 00000000000cd000 00007f79358ca000 joined      3
00000000000cd000 00000000000e8000 00007f79358cd000 joined      3
00000000000e8000 00000000000f0000 00007f79358e8000 joined      3
00000000000f0000 0000000000100000 00007f79358f0000 joined      3
0000000000100000 0000000080000000 00007f7935900000 joined      3
00000000fd000000 00000000fe000000 00007f7934200000 added       4
00000000fffc0000 0000000100000000 00007f7935600000 added       5
Python Exception <class 'gdb.error'> You can't do that without a process to debug.:
Error occurred in Python command: You can't do that without a process
to debug.

Replace the object_resolve_path_type() function call with a local
volatile variable.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

hw/misc/vmcoreinfo.c

@@ -35,6 +35,8 @@ static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
 {
     VMCoreInfoState *s = VMCOREINFO(dev);
     FWCfgState *fw_cfg = fw_cfg_find();
+    /* for gdb script dump-guest-memory.py */
+    static VMCoreInfoState * volatile vmcoreinfo_state G_GNUC_UNUSED;
 
     /* Given that this function is executing, there is at least one VMCOREINFO
      * device. Check if there are several.
@@ -56,6 +58,7 @@ static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
                              &s->vmcoreinfo, sizeof(s->vmcoreinfo), false);
 
     qemu_register_reset(vmcoreinfo_reset, dev);
+    vmcoreinfo_state = s;
 }
 
 static const VMStateDescription vmstate_vmcoreinfo = {

scripts/dump-guest-memory.py

@@ -546,8 +546,7 @@ def phys_memory_read(self, addr, size):
         return None
 
     def add_vmcoreinfo(self):
-        vmci = '(VMCoreInfoState *)' + \
-               'object_resolve_path_type("", "vmcoreinfo", 0)'
+        vmci = 'vmcoreinfo_realize::vmcoreinfo_state'
         if not gdb.parse_and_eval("%s" % vmci) \
            or not gdb.parse_and_eval("(%s)->has_vmcoreinfo" % vmci):
             return