qemu: pass numa node binding preferences to qemu

Currently, we only bind the whole QEMU domain to memory nodes
specified in nodemask altogether.  That, however, doesn't make much
sense when one wants to control from where the memory for particular
guest nodes should be allocated.  QEMU allows us to do that by
specifying 'host-nodes' parameter for the 'memory-ram' object, so
let's use that.

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