src: Move _virDomainMemoryDef target nodes into an union

src: Move _virDomainMemoryDef target nodes into an union

The _virDomainMemoryDef struct is getting a bit messy. It has
various members and only some of them are valid for given model.
Worse, some are re-used for different models. We tried to make
this more bearable by putting a comment next to each member
describing what models the member is valid for, but that gets
messy too.

Therefore, do what we do elsewhere: introduce an union of structs
and move individual members into their respective groups.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>