KVM: nVMX: initialize descriptor cache fields in prepare_vmcs02_full

This part is separate for ease of review, because git prefers to move
prepare_vmcs02 below the initial long sequence of vmcs_write* operations.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>