Sort the help info shown in monitor at runtime

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 <[email protected]>
Signed-off-by: Wayne Xia <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>