Adopt stable `libxenmanage` instead of `xenctrl` glue

[last-genius]

See the patch series around 42630ced upstream, and further related improvements posted at https://lore.kernel.org/xen-devel/20250314121835.1879-1-jgross@suse.com/.

This would allow us to

1. drop the whole xenctrl plugin infrastructure from here and upstream for 4.21+
2. do true multithreading, since xenctrl is unsafe to use in a multithreaded context (though we do not use known unsafe things, it might still be)

Preferably, to avoid forking oxenstored again, backport these patches to 4.20 (used in XenServer 9)

I have the bindings draft here: https://github.com/last-genius/oxenstored/tree/private/asultanov/drop-xenctrl