memory: add -machine dump-guest-core=on|off

Add a new '[,dump-guest-core=on|off]' option to the '-machine' option. When
'dump-guest-core=off' is specified, guest memory is omitted from the core dump.
The default behavior continues to be to include guest memory when a core dump is
triggered. In my testing, this brought the core dump size down from 384MB to 6MB
on a 2GB guest.

Is anything additional required to preserve this setting for migration or
savevm? I don't believe so.

Changelog:
v3:
    Eliminate globals as per Anthony's suggestion
    set no dump from qemu_ram_remap() as well
v2:
    move the option from -m to -machine, rename option dump -> dump-guest-core

Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/qemu-config.c b/qemu-config.c
index 5c3296b..6700de0 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -595,6 +595,10 @@
             .name = "dt_compatible",
             .type = QEMU_OPT_STRING,
             .help = "Overrides the \"compatible\" property of the dt root node",
+        }, {
+            .name = "dump-guest-core",
+            .type = QEMU_OPT_BOOL,
+            .help = "Include guest memory in  a core dump",
         },
         { /* End of list */ }
     },