machine: query dump-guest-core machine property

Running
    qemu-bin ... -machine pc,dump-guest-core=on
leads to crash:
    x86_64-softmmu/qemu-system-x86_64 -machine pc,dump-guest-core=on
    qemu-system-x86_64: qemu/util/qemu-option.c:387: qemu_opt_get_bool_helper:
    Assertion `opt->desc && opt->desc->type == QEMU_OPT_BOOL' failed.  Aborted
    (core dumped)

This happens because the commit e79d5a6 ("machine: remove qemu_machine_opts
global list") removed the global option descriptions and moved them to
MachineState's QOM properties.

Fix this by querying machine properties through designated wrappers.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/exec.c b/exec.c
index b44a33c..f55270b 100644
--- a/exec.c
+++ b/exec.c
@@ -26,6 +26,7 @@
 #include "cpu.h"
 #include "tcg.h"
 #include "hw/hw.h"
+#include "hw/boards.h"
 #include "hw/qdev.h"
 #include "qemu/osdep.h"
 #include "sysemu/kvm.h"
@@ -1250,8 +1251,7 @@
     int ret;
 
     /* Use MADV_DONTDUMP, if user doesn't want the guest memory in the core */
-    if (!qemu_opt_get_bool(qemu_get_machine_opts(),
-                           "dump-guest-core", true)) {
+    if (!machine_dump_guest_core(current_machine)) {
         ret = qemu_madvise(addr, size, QEMU_MADV_DONTDUMP);
         if (ret) {
             perror("qemu_madvise");