KVM: nVMX: Don't rewrite GUEST_PML_INDEX during nested VM-Entry

Emulation of GUEST_PML_INDEX for a nested VMM is a bit weird.  Because
L0 flushes the PML on every VM-Exit, the value in vmcs02 at the time of
VM-Enter is a constant -1, regardless of what L1 thinks/wants.

Fixes: 09abe320 ("KVM: nVMX: split pieces of prepare_vmcs02() to prepare_vmcs02_early()")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>