Sort the help info shown in monitor at runtime

This patch would try sort the command list in monitor at runtime. As a result,
command help and help info would show a more friendly sorted command list.
For eg:
(qemu)help
acl_add
acl_policy
acl_remove
acl_reset
acl_show
balloon
block_passwd
...
the command list is sorted.

v3: using qsort function to sort the command list.

Tested-by: Wenyi Gao <wenyi@linux.vnet.ibm.com>
Signed-off-by: Wayne Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>