Re: [PATCH 2/4] qemu_capabilities: Detect memory-backend-*.prealloc-threads property

The prealloc-threads is property of memory-backend class which is parent to the other three classes memory-backend-{ram,file,memfd}. Therefore the property is present for all, or none if QEMU is older than v5.0.0-rc0~75^2~1^2~3 which introduced the property. Anyway, the .reserve property is the same story, and we chose memory-backend-file to detect it, so stick with our earlier decision and use the same backend to detect this new property. Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt;