monitor: reuse user_creatable_add_opts() instead of user_creatable_add()

Simplify code by dropping ~57LOC by merging user_creatable_add()
into user_creatable_add_opts() and using the later from monitor.
Along with it allocate opts_visitor_new() once in user_creatable_add_opts().

As result we have one less API func and a more readable/simple
user_creatable_add_opts() vs user_creatable_add().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1484052795-158195-3-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>