gdbstub: init mon_chr through qemu_chr_alloc

This patch initializes monitor for gdbstub with the qemu_chr_alloc function
instead of just allocating the memory. Initialization function call
is required, because it also creates chr_write_lock mutex, which is used
when writing to this character device.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 462efe9e530e22b1b60aaf01716e1423cd94302c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

gdbstub.c

@@ -1707,7 +1707,7 @@ int gdbserver_start(const char *device)
         qemu_add_vm_change_state_handler(gdb_vm_state_change, NULL);
 
         /* Initialize a monitor terminal for gdb */
-        mon_chr = g_malloc0(sizeof(*mon_chr));
+        mon_chr = qemu_chr_alloc();
         mon_chr->chr_write = gdb_monitor_write;
         monitor_init(mon_chr, 0);
     } else {