KVM: SVM: optimize nested vmrun

Only copy the necessary parts of the vmcb save area on vmrun and save
precious time.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>