introduce a new monitor command 'dump-guest-memory' to dump guest's memory

The command's usage:
   dump-guest-memory [-p] protocol [begin] [length]
The supported protocol can be file or fd:
1. file: the protocol starts with "file:", and the following string is
   the file's path.
2. fd: the protocol starts with "fd:", and the following string is the
   fd's name.

Note:
  1. If you want to use gdb to process the core, please specify -p option.
     The reason why the -p option is not default is:
       a. guest machine in a catastrophic state can have corrupted memory,
          which we cannot trust.
       b. The guest machine can be in read-mode even if paging is enabled.
          For example: the guest machine uses ACPI to sleep, and ACPI sleep
          state goes in real-mode.
  2. If you don't want to dump all guest's memory, please specify the start
     physical address and the length.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qmp-commands.hx

@@ -601,6 +601,42 @@ Example:
                     "port": 1234 } }
 <- { "return": {} }
 
+EQMP
+
+    {
+        .name       = "dump-guest-memory",
+        .args_type  = "paging:b,protocol:s,begin:i?,end:i?",
+        .params     = "-p protocol [begin] [length]",
+        .help       = "dump guest memory to file",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = qmp_marshal_input_dump_guest_memory,
+    },
+
+SQMP
+dump
+
+
+Dump guest memory to file. The file can be processed with crash or gdb.
+
+Arguments:
+
+- "paging": do paging to get guest's memory mapping (json-bool)
+- "protocol": destination file(started with "file:") or destination file
+              descriptor (started with "fd:") (json-string)
+- "begin": the starting physical address. It's optional, and should be specified
+           with length together (json-int)
+- "length": the memory size, in bytes. It's optional, and should be specified
+            with begin together (json-int)
+
+Example:
+
+-> { "execute": "dump-guest-memory", "arguments": { "protocol": "fd:dump" } }
+<- { "return": {} }
+
+Notes:
+
+(1) All boolean arguments default to false
+
 EQMP
 
     {