KVM: SVM: copy only necessary parts of the control area on vmrun/vmexit

The vmcb control area contains more then 800 bytes of reserved fields
which are unnecessarily copied. Fix this by introducing a copy
function which only copies the relevant part and saves 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>