qemu-img: fix segment fault when the image format is qed

[root@f15 qemu]# qemu-img info /home/zwu/work/misc/rh6.img
image: /home/zwu/work/misc/rh6.img
file format: qed
virtual size: 4.0G (4294967296 bytes)
disk size: 1.2G
cluster_size: 65536
Segmentation fault (core dumped)

Today when i were fixing another issue, i found this issue; After simple
investigation, i found that the required clock vm_clock is not created
for qemu tool.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1655,6 +1655,8 @@ int main(int argc, char **argv)
     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)) {