conf: refactor checking for unsupported memory devices

conf: refactor checking for unsupported memory devices

Introduce a helper to check supported device and domain config and move
the memory hotplug checks to it.

The advantage of this approach is that by default all new features are
considered unsupported by all hypervisors unless specifically changed
rather than the previous approach where every hypervisor would need to
declare that a given feature is unsupported.