KVM: PPC: Book3S HV: XIVE: Fix the enforced limit on the vCPU identifier

When a vCPU is connected to the KVM device, it is done using its vCPU
identifier in the guest. Fix the enforced limit on the vCPU identifier
by taking into account the SMT mode.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Tested-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>