KVM: nVMX: Cache shadow vmcs12 on VMEntry and flush to memory on VMExit

This is done is done as a preparation to VMCS shadowing emulation.

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>