conf: Validate virDomainMemoryDef::targetNode

conf: Validate virDomainMemoryDef::targetNode

Almost all of memory models we currently support allow setting
virDomainMemoryDef::targetNode so that the memory module is
associated with given guest NUMA node. And we do have a check
whether the requested node is within bounds, but it's executed
only when building QEMU's cmd line. Move it into validation
phase.

While this commit is moving the validation to a place that does
not validate all the possible code paths, it's okay, because only
the explicit memory device has user-configurable target node
which could break the assumption.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>