KVM: nVMX: Fix returned value of MSR_IA32_VMX_VMCS_ENUM

Many real CPUs get this wrong as well, but ours is totally off: bits 9:1
define the highest index value.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>