KVM: SVM: Emulate read/write access to cr8

This patch adds code to emulate the access to the cr8 register to the x86
instruction emulator in kvm.  This is needed on svm, where there is no
hardware decode for control register access.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Markus Rechberger <markus.rechberger@amd.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>