qom: Factor out helpers from user_creatable_print_help()

qom: Factor out helpers from user_creatable_print_help()

This creates separate helper functions for printing a list of user
creatable object types and for printing a list of properties of a given
type. This will allow using these parts without having a QemuOpts.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201007164903.282198-3-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>