Add GDB qAttached support

With this patch QEMU handles qAttached request from gdb. When QEMU
replies 1, GDB sends a "detach" command at the end of a debugging
session otherwise GDB sends "kill".

The default value for qAttached is 1 on system emulation and 0 on user
emulation.

Based on original version by Fabien Chouteau.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -41,6 +41,12 @@
 #include "qemu/sockets.h"
 #include "sysemu/kvm.h"
 
+#ifdef CONFIG_USER_ONLY
+#define GDB_ATTACHED "0"
+#else
+#define GDB_ATTACHED "1"
+#endif
+
 static inline int target_memory_rw_debug(CPUState *cpu, target_ulong addr,
                                          uint8_t *buf, int len, bool is_write)
 {
@@ -1195,6 +1201,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
             put_packet_binary(s, buf, len + 1);
             break;
         }
+        if (is_query_packet(p, "Attached", ':')) {
+            put_packet(s, GDB_ATTACHED);
+            break;
+        }
         /* Unrecognised 'q' command.  */
         goto unknown_command;