numa: Move NUMA declarations from sysemu.h to numa.h

Not all sysemu.h users need the NUMA declarations, and keeping them in a
separate file makes it easier to see what are the interfaces provided by
numa.c.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>