qemu_capabilities: Detect memory-backend-*.prealloc-threads property

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@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>