pnv/xive2: Fail VST entry address computation if table has no VSD

pnv/xive2: Fail VST entry address computation if table has no VSD

Fail VST entry address computation if firmware doesn't define a descriptor
for one of the Virtualization Structure Tables (VST), there's no point in
trying to compute the address of its entry.  Abort the operation and log
an error.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Kowal <kowal@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>