fix #4984: manager: add service to migration-target usage only if online

fix #4984: manager: add service to migration-target usage only if online

Otherwise, when using the 'basic' plugin, this would lead to
auto-vilification of the $target node in the Perl hash tracking the
usage and it would wrongly be considered online when selecting the
recovery node.

The 'static' plugin was not affected, because it would check and warn
before adding usage to a node that was not registered with add_node()
first. Doing the same in the 'basic' plugin will be done by another
patch.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
 [ TL: shorten commit message subject ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>