conf: move NUMA capabilities into self contained object

conf: move NUMA capabilities into self contained object

The NUMA cells are stored directly in the virCapsHostPtr
struct. This moves them into their own struct allowing
them to be stored independantly of the rest of the host
capabilities. The change is used as an excuse to switch
the representation to use a GPtrArray too.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>