nVMX: Keep arch.pat in sync on L1-L2 switches

When asking vmx to load the PAT MSR for us while switching from L1 to L2
or vice versa, we have to update arch.pat as well as it may later be
used again to load or read out the MSR content.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Tested-by: Arthur Chunqi Li <yzt356@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>