Monitor: Introduce cmd_new_ret()

Monitor: Introduce cmd_new_ret()

In order to implement the new error handling and debugging
mechanism for command handlers, we need to change the cmd_new()
callback to return a value.

This commit introduces cmd_new_ret(), which returns a value and
will be used only temporarily to handle the transition from
cmd_new().

That is, as soon as all command handlers are ported to cmd_new_ret(),
it will be renamed back to cmd_new() and the new error handling
and debugging mechanism will be added on top of it.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>