target-ppc: Use #define instead of opencoding SLB valid bit

Use SLB_ESID_V instead of (1 << 27) in the code

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
