KVM: VMX: Simplify pdptr and cr3 management

Instead of reading the PDPTRs from memory after every exit (which is slow
and wrong, as the PDPTRs are stored on the cpu), sync the PDPTRs from
memory to the VMCS before entry, and from the VMCS to memory after exit.
Do the same for cr3.

Signed-off-by: Avi Kivity <avi@redhat.com>