tools: initialize main loop before block layer

Tools were broken because they initialized the block layer while
qemu_aio_context was still NULL.

Reported-by: malc <av1474@comtv.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>
diff --git a/qemu-img.c b/qemu-img.c
index b17bddd..e29e01b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2001,14 +2001,13 @@
 
     error_set_progname(argv[0]);
 
+    qemu_init_main_loop();
     bdrv_init();
     if (argc < 2)
         help();
     cmdname = argv[1];
     argc--; argv++;
 
-    qemu_init_main_loop();
-
     /* find the command */
     for(cmd = img_cmds; cmd->name != NULL; cmd++) {
         if (!strcmp(cmdname, cmd->name)) {