pc: Move enforce_aligned_dimm to PCMachineClass

enforce_aligned_dimm never changes after the machine is
initialized, so it can be simply set in PCMachineClass like all
the other compat fields.

Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
